LIBRARY 

OF  THE 

UNIVERSITY  OF  CALIFORNIA. 

GIF^T  Ot^ 


Class 


SUMMARY 


OF 


RESEARCHES  IN  SOUND 


CONDUCTED  IN  THE   SERVICE   OF 


THE  UNITED  STATES  LIGHT-HOUSE  BOARD, 


BY 


JOSEPH   HENRY, 


DURING    THE    YEARS    1865    TO    1877 


[FROM  THE  SMITHSONIAN  REPORT  FOR  1878. J 


WASHINGTON: 

GOVERNMENT  PRINTING  OFFICE 
1879. 


(I  'V 


RESEARCHES  IN  SOUND: 

WITH  SPECIAL  KEFERENCE  TO  FOG-SIGNALING. 


BY  JOSEPH  HENRY. 
[FROM  THE  ANNUAL  REPORTS  OF  THE  U.  S.  LIGHT-HOUSE  BOARD.] 


PREFATORY   NOTE. 

[The  series  of  investigations  undertaken  by  the  late  Professor  Henry 
in  the  interest  of  the  light-house  service,  embracing  not  only  observa- 
tions more  than  usually  laborious  and  extended,  with  regard  to  the 
atmospheric  conditions  affecting  the  propagation  of  sound  to  a  distance,, 
but  varied  and  elaborate  experimental  inquiries,  as  well,  with  reference- 
to  the  most  efficient  character  and  form  of  sonorous  instruments  for 
fog-signaling  purposes,  commenced  as  far  back  as  the  year  1865,  and 
continued  to  the  last  year  of  his  life. 

These  important  investigations,  though  in  the  language  of  an  official 
report,  they  "have  resulted  in  giving  us  a  fog-signal  service  conceded 
to  be  the  best  in  the  world,"*  have  hitherto  received  so  little  publicity 
and  attention,  owing  to  the  purely  official  character  and  channel  of  their 
presentation,  that  their  collection  and  republication  here  (in  advance  of 
of  a  possible  edition  of  Henry's  collected  works),  appears  to  be  called 
for  in  the  interests  of  science,  as  well  as  of  a  just  appreciation  of  the 
value  of  his  prolonged  researches. 

The  first  part,  extracted  from  the  Appendix  to  the  Light  House  Ee- 
port  for  1874,  comprises  a  preliminary  statement  and  an  account  of  ob- 
servations and  experiments  made  by  the  author,  from  1865,  at  New 

*  Executive  Document  No.  94,  Forty-fifth  Congress,  second  session,  Senate,  p.  2. 
This  is  a  report  to  the  Hon.  Secretary,  of  the  Treasury,  made  since  Professor  Henry's 
death.  To  a  similar  effect,  may  be  quoted  an  official  statement  made  five  years 
earlier.  In  1873,  Major  George  H.  Elliot,  of  the  Light-House  Board,  commissioned  to 
make  a  tour  of  inspection  of  European  light-house  establishments,  presented  the  re- 
sults of  his  observations  abroad  in  a  very  able  and  elaborate  report,  published  by  the 
Senate  in  272  octavo  pages,  with  numerous  illustrations.  In  his  preliminary  report  to 
the  Board,  dated  September  17,  1873,  he  concludes,  that  while  there  are  "  many  details 
of  construction  and  administration  which  we  can  adopt  with  advantage,"  (from  the 
British  and  French  light-house  systems,)  "there  are  many  in  which  we  excel.  Our 
shore  fog-signals,  particularly,  are  vastly  superior,  both  in  number  and  power."' 
(Executive  Document  No.  54,  Forty-third  Congress,  first  session;  Senate,  p.  12.) 

455 

130004 


456  RESEARCHES   IN   SOUND. 

Haven,  Conn.,  to  1872,  at  Portland,  Me.  The  second  part  is  a  commu- 
nication made  to  the  "Philosophical  Society  of  Washington n  (of  which 
he  was  the  president)  December  11,  1872,  embracing  a  discussion  of 
some  abnormal  phenomena  of  sound,  and  is  extracted  from  the  Bulletin 
of  the  Society,  vol.  ii,  appendix  ix.  The  third  part,  forming  the  latter 
portion  of  the  Appendix  to  the  Light-House  Report  for  1874,  comprises 
investigations  extending  from  1873,  at  Whitehead  Station,  off  the  coast 
of  Maine,  to  1874,  at  Sandy  Hook,  New  Jersey.  The  fourth  part,  extracted 
from  the  Appendix  to  the  Light-House  Eeport  for  1875,  comprises  his 
investigations  for  that  year.  And  the  fifth  part,  from  the  Appendix  to 
the  Light-House  Eeport  of  1877,  embraces  his  latest  observations  dur- 
ing September  and  October  of  the  year  1877. 

These  papers  necessarily  are  more  fragmentary,  and  at  the  same  time 
involve  more  recapitulation  than  would  have  been  the  case,  could  they 
have  received  the  revision  of  their  distinguished  and  lamented  author. 
It  has  however  been  considered  more  just  to  reproduce  these  contribu- 
tions in  this  form,  than  to  attempt  either  a  compilation  of  extracts,  or  a 
condensation  of  their  substance  in  the  form  of  an  abstract.] 

S.  F.  BAIRD. 


RESEARCHES    IN    SOUND, 


INTRODUCTION.* 

• 

FOG. 

Among  the  impediments  to  navigation  none  perhaps  are  more  to  be 
dreaded  than  those  which  arise  from  fogs,  and  consequently  the  nature 
of  this  impediment  and  the  means  which  may  be  devised  for  obviating 
it  are  objects  of  great  interest  to  the  mariner.  Fogs  are  in  all  cases 
produced  when  cold  air  is  mingled  with  warm  air  saturated  with  moist- 
ure. In  this  case  the  invisible  vapor  of  the  warmer  air  is  condensed  by 
the  cold  into  minute  particles  of  liquid  water,  which,  by  their  immense 
number  and  multiplicity  of  reflecting  surfaces,  obstruct  the  rays  of  light 
in  the  same  way  that  a  piece  of  transparent  glass  when  pounded  becomes 
almost  entirely  opaque  and  is  seen  by  reflection  as  a  white  mass.  So 
greatly  does  a  dense  fog  obstruct  light,  that  the  most  intense  artifi- 
cial illumination,  such  as  that  produced  by  the  combustion  of  magnesium, 
by  the  burning  of  oxygen  and  hydrogen  in  contact  with  lime,  and  that 
produced  between  the  charcoal  points  of  a  powerful  electrical  apparatus, 
are  entirely  obscured  at  comparatively  short  distances.  Even  the  light 
of  the  sun,  which  is  far  more  intense  than  that  of  any  artificial  illumina- 
tion, is  so  diminished  by  a  single  mile  of  dense  fog  that  the  luminary 
itself  becomes  invisible.  Eecourse  must  therefore  be  had  to  some  other 
means  than  that  of  light  to  enable  the  mariner  to  recognize  his  position 
on  approaching  the  coast  when  the  land  is  obscured  by  fog. 

The  only  means  at  present  known  for  obviating  the  difficulty  is  that 
of  employing  powerful  sounding  instruments  which  may  be  heard  at  a 
sufficient  distance  through  the  fog  to  give  timely  warning  of  impending- 
danger.  Investigations  therefore  as  to  the  nature  of  sound  and  its 
applications  to  fog-signals  become  an  important  object  to  those  in  charge 
of  aids  to  navigation.  Such  investigations  are  of  special  importance  in 
connection  with  the  light-house  service  of  the  United  States.  The 
northeastern  coast  of  the  United  States  on  the  Atlantic,  and  the  entire 
western  coast  on  the  Pacific,  included  in  our  territory,  are  subject, 
especially  during  the  summer  months,  to  dense  fogs,  which  greatly  im- 
pede navigation,  as  well  as  endanger  life  and  property. 

The  origin  of  the  fogs  on  our  coast  is  readily  explained  by  reference 
to  a  few  simple  principles  of  physical  geography.  In  the  Atlantic  Ocean 
there  exists  a  current  of  warm  water  proceeding  from  the  Gulf  of  Mex- 
ico, between  Cuba  and  Florida,  which  flows  along  our  coast  to  the  lati- 

*  From  the  Report  of  the  Light-House  Board,  for  1874. 

457 


458  RESEARCHES    IN    SOUND. 

tude  of  about  35°,  and  then  turning  gradually  to  the  eastward,  crosses  the 
Atlantic  and  impinges  against  the  coast  of  Northern  Europe.  Through- 
out its  entire  course,  on  account  of  the  immense  capacity  of  water  for 
heat,  the  temperature  of  the  stream  is  greater  than  that  of  the  ocean  on 
either  side.  In  addition  to  this  stream,  the  Atlantic  Ocean  is  traversed 
by  another  current  of  an  entirely  opposite  character,  one  of  cold  water, 
which,  coming  from  the  arctic  regions  down  Davis's  Strait,  is  thrown,  by 
the  rotation  of  the  earth,  against  our  coast,  passing  between  it  and  the 
Gulf-stream,  and  sinking  under  the  latter  as  it  approaches  the  southern 
extremity  of  the  United  States. 

These  conditions  are  those  most  favorable  to  the  production  of  fogs, 
since  whenever  the  warm  air,  surcharged  with  moisture,  is  blown  from 
the  Gulf-stream  over  the  arctic  current  and  mingles  with  the  cold  air 
of  the  latter,  a  precipitation  of  its  vapor  takes  place  in  the  form  of  fog. 
Hence,  especially  in  summer,  when  the  wind  in  the  eastern  part  of  the 
United  States  is  in  a  southeasterly  direction,  fogs  prevail.  As  we  pro- 
ceed southerly  along  the  coast,  the  fog-producing  winds  take  a  more 
easterly  direction. 

A  somewhat  similar  circulation  in  the  Pacific  Ocean  produces  fogs  on 
the  western  coast  of  the  United  States.  In  this  ocean  a  current  of  warm 
water,  starting  from  the  equatorial  regions,  passes  along  the  shores  of 
China  and  Japan,  and,  following  the  general  trend  of  the  coast,  turns 
eastward  and  continues  along  our  shore.  The  northern  part  of  this 
current  being  warmer  than  the  ocean  through  which  it  passes,  tends  to 
produce  dense  fogs  in  the  region  of  the  Aleutian  Islands  and  tlie  coast 
of  Alaska.  As  this  current  descends  along  the  American  coast  into 
lower  latitudes  it  gradually  loses  its  warmth,  and  soon  assumes  the 
character,  in  regard  to  the  water  through  which  it  passes,  of  a  compara- 
tively colder  stream;  and  to  this  cause  we  would  attribute  the  preva- 
lence of  fogs  on  the  coast  of  Oregon  and  California,  which  are  most 
prevalent  during  the  spring  and  early  summer,  with  wind  from  the 
northwest  and  west. 

From  what  has  been  said,  it  is  evident  that  the  fogs  in  the  Aleutian 
Islands  occur  chiefly  in  summer,  when  southwesterly  winds  prevail  and 
mingle  the  moist  air  from  the  warm  current  with  the  colder  air  of  the 
more  northerly  latitude.  In  winter,  the  wind  being  from  the  north 
chiefly,  the  moist  air  is  driven  in  an  opposite  direction,  and  dense  fogs 
therefore  at  this  season  do  not  prevail. 

In  regard  to  the  fogs  on  the  coast  of  Maine,  the  following  interesting 
facts  were  furnished  me  by  the  late  Dr.  Stiinpson,  formerly  of  the  Smith- 
sonian Institution  and  of  the  Chicago  Academy  of  Sciences,  who  had 
much  experience  as  to  the  weather  during  his  dredging  tor  marine  speci- 
mens of  natural  history  in  the  region  of  Grand  Manan  Island,  at  the 
entrance  of  the  Bay  of  Fundy. 

aSo  sharply  marked,"  says  Dr.  Stimpson,  "is  the  difference  of  tem- 
perature of  the  warm  water  from  the  Gulf-stream  and  that  of  the  polar 


RESEARCHES    IN    SOUND.  459 

current,  that  in  sailing  in  some  cases  only  a  few  lengths  of  a  ship  the 
temperature  of  the  water  will  change  from  70°  to  50°.  The  fog  fre- 
quently comes  rolling  in  with  the  speed  of  a  race-horse  5  in  some  cases 
while  dredging,  happening  to  turn  my  eyes  to  the  south,  a  bank  of  fog 
has  been  seen  approaching  with  such  rapidity  that  there  was  scarcely 
time  in  which  to  take  compass-bearing  of  some  object  on  shore  by  which 
to  steer,  before  I  would  be  entirely  shut  in,  perhaps  for  days  together." 
He  also  mentions  the  fact  that  it  frequently  happened  during  a  warm 
day,  while  a  dense  fog  existed  some  distance  from  the  shore,  close  in  to 
the  latter  there  would  be  a  space  entirely  clear;  this  was  probably  due 
to  the  reflection  and  radiation  of  the  heat  from  the  land,  which  converted 
the  watery  particles  into  invisible  vapor. 

Dr.  Stimpson  has  also  noticed  another  phenomenon  of  some  interest. 
"When  a  dense  fog,  coming  in  regularly  from  the  sea,  reaches  the  land, 
it  gradually  rises  in  the  atmosphere  and  forms  a  heavy,  dark  cloud,  which 
is  frequently  precipitated  in  rain."  This  rising  of  fog  is  not  due,  accord- 
ing to  the  doctor,  to  a  surface- wind  from  the  west  pressing  under  it  and 
buoying  it  upward,  since  the  wind  at  the  time  is  from  the  ocean.  It  is 
probably  due  to  the  greater  heat  of  the  land  causing  an  upward  cur- 
rent, which,  when  once  started,  by  its  inertia  carries  the  cloud  up  to  a 
region  of  lower  temperature,  and  hence  the  precipitation.  The  height 
of  the  fog  along  the  coast  is  not  usually  very  great,  and  can  be  frequently 
overlooked  from  the  mast-head.  The  deception  as  to  size  and  distance 
of  objects  as  seen  in  a  fog  is  also  a  remarkable  phenomenon  when  ob- 
served for  the  first  time.  A  piece  of  floating  wood  at  a  little  distance  is 
magnified  into  a  large  object,  and  after  much  experience  the  doctor  was 
not  able  to  overcome-the  delusion.  It  is  said  that  the  sailors  in  the  Bay 
of  Fuudy  prefer  of  two  evils  a  fog  that  remains  constant  in  density  to 
one  that  is  variable,  although  the  variation  may  be  toward  a  greater 
degree  of  lightness,  on  account  of  the  varying  intensity  producing  a 
varied  and  erroneous  impression  of  the  size  and  distance  of  the  object 
seen  through  it.  It  is  also  his  impression  that  sound  can  be  heard  as 
well  during  fog  as  in  clear  weather,  although  there  is  a  delusion  even 
in  this,  since  the  source  of  sound,  when  seen,  appears  at  a  greater  dis- 
tance than  in  a  clear  atmosphere,  and  hence  the  sound  itself  would 
appear  to  be  magnified. 

Fogs  also  exist  on  the  Mississippi,  especially  on  the  lower  portion  of 
the  river.  They  are  of  two  classes,  those  which  result  from  the  cooling 
of  the  earth,  particularly  during  the  summer  in  clear  nights,  with  wind 
probably  from  a  northerly  direction,  followed  by  a  gentle,  warm  wind 
from  the  south  surcharged  with  moisture,  and  the  other  induced  by  the 
water  of  the  river,  which,  coming  from  melting  snow  of  northern  regions, 
is  colder  than  the  air  in  the  vicinity.  The  air  over  the  river  being  thus 
cooled  below  the  temperature  of  a  gentle  wind  from  the  south,  the 
moisture  of  the  latter  is  precipitated.  This  fog,  which  occurs  in  the  last 
of  winter,  during  the  spring,  and  beginning  of  summer,  is  very  dense, 


460  RESEARCHES    IN    SOUND. 

but  is  confined  entirely  to  the  atmosphere  above  the  river,  while  the 
other  class  of  fog  exists  over  the  land  as  well. 

FOG- SIGNALS. 

The  importance  of  fog-signals  as  aids  to  navigation,  especially  on  the 
northeastern  portion  of  our  coast,  of  which  the  shore  is  exceedingly  bold 
and  to  the  approach  of  which  the  sounding-line  gives  no  sure  indication, 
has  been  from  the  first  an  object  of  special  attention. 

At  the  beginning  of  the  operations  of  the  Light-House  Board  such 
instruments  were  employed  for  producing  sound  as  had  been  used  in 
other  countries ;  these  consisted  of  gongs,  bells,  guns,  horns,  &c.  The 
bells  were  actuated  by  clock-machinery,  which  was  wound  up  from  time 
to  time  and  struck  at  intervals  of  regular  sequence  by  which  their  posi- 
tion might  be  identified.  The  machinery,  however,  by  which  these  bells 
were  struck  was  of  a  rude  character  and  exceedingly  wasteful  of  power, 
the  weight  continuing  to  descend  during  the  whole  period  of  operation, 
including  the  successive  intervals  of  silence.  This  defect  was  remedied 
by  the  invention  of  Mr.  Stevens,  who  introduced  an  escapement  arrange- 
ment, similar  to  that  of  a  clock,  which  is  kept  in  motion  by  a  small 
weight,  a  larger  one  being  brought  into  operation  only  during  the 
instant  of  striking. 

Bell-buoys  were  also  introduced  at  various  points.  These  consisted 
of  a  bell  supported  on  a  water-tight  vessel  and  rung  by  the  oscillation 
of  the  waves,  but  all  contrivances  of  this  kind  have  been  found  to  be 
untrustworthy;  the  sound  which  they  emit  is  comparatively  of  feeble 
character,  can  be  heard  at  but  a  small  distance,  and  is  frequently  ineffi- 
cient during  a  fog  which  occurs  in  calm  weather*  Besides  this,  auto- 
matic fog- signals  are  liable  to  be  interfered  with  by  ice  in  northern 
positions,  and  in  all  sections  to  derangement  at  times  when  no  substi- 
tute can  be  put  in  their  place,  as  can  be  in  the  cases  of  the  bells  rung 
by  machinery  under  the  immediate  control  of  keepers.  A  signal  which 
is  liable  to  be  interrupted  in  its  warnings  is  worse  than  no  signal,  since 
its  absence  may  give  confidence  of  safety  in  the  midst  of  danger,  and 
thus  prevent  the  necessary  caution  which  would  otherwise  be  employed. 

Guns  have  been  employed  on  the  United  States  coast,  first  under  the 
direction  of  General  Bates,  engineer  of  the  twelfth  district,  at  Point 
Bonita,  San  Francisco  Bay,  California.  The  gun  at  this  station  con- 
sisted of  a  24-pounder,  furnished  by  the  War  Department.  The  neces- 
sary arrangements  being  made,  by  the  construction  of  a  powder-house, 
and  laying  of  a  platform,  and  employment  of  a  gunner,  notice  to  mari- 
ners was  given  that  after  the  8th  of  August,  1856,  a  signal- gun  would 
be  fired  every  hour  and  half-hour,  night  and  day,  during  foggy  or  thick 
weather.  The  first  year,  with  the  exception  of  eighty-eight  foggy  days, 
omitted  for  want  of  powder,  1,390  rounds  were  fired.  These  consumed 
5,560  pounds  of  powder,  at  a  cost  of  $1,487,  pay  of  gunner  and  incident- 
als excluded.  The  following  3<ear  the  discharges  were  1,&S2,  or  about 


RESEARCHES    IN    SOUND.  461 

one-eleventh  of  the  number  of  hours  and  half-hours  of  the  whole  time. 
The  fog-gun  was  found  to  answer  a  useful  purpose ;  vessels  by  the  help 
of  it  alone  having  come  into  the  harbor  during  a  fog  at  night,  as  well  as 
in  the  day,  that  otherwise  could  not  possibly  have  entered.  This  signal 
was  continued  until  it  was  superseded  by  a  bell  boat.  A  gun  was  also 
used  at  West  Quoddy  Head,  near  the  extreme  eastern  part  of  Maine. 
It  consisted  of  a  short  piece,  or  carronade,  5  feet  long,  with  a  bore  of  5J 
inches,  charged  with  four  pounds  of  blasting-powder.  The  powder  was 
made  up  in  cartridges  and  kept  in  chests  in  the  work-house.  The  gun 
was  only  fired  on  foggy  days,  when  the  steamboat  running  between  Boston 
and  Saint  John,  New  Brunswick,  was  approaching  the  light-house  from 
the  former  place.  In  going  in  the  other  direction  the  signal  was  not  so 
much  required,  because  in  the  former  case  (of  approach)  the  vessel  had 
been  for  some  time  out  of  sight  of  land,  and  consequently  its  position 
could  not  be  so  well  known.  The  firing  was  commenced  with  the  hear- 
ing of  the  steamer's  whistle  as  she  was  approaching,  and  as  the  wind 
during  the  fog  at  this  place  is  generally  from  the  south,  the  steamer 
could  be  heard  five  or  six  miles.  The  firing  was  continued  as  frequently 
as  the  gun  could  be  loaded  until  the  steamer  answered  by  a  signal  of 
three  puff's  of  its  whistle.  The  number  of  discharges  was  from  one  to 
six,  the  latter  exhausting  a  keg  of  powder  valued  at  $8.  The  keeper  of 
the  light-house  acted  as  gunner,  without  compensation  other  than  his 
salary.  The  cost  of  powder  was  paid  by  the  steamboat  company.  The 
report  of  the  gun  was  heard  from  two  to  six  miles. 

This  signal  has  been  abandoned, — because  of  the  danger  attending  its 
use, — the  length  of  the  intervals  between  the  successive  explosions, — 
and  the  brief  duration  of  the  sound,  which  renders  it  difficult  to  deter- 
mine with  accuracy  its  direction. 

The  lamented  General  Bache,  of  the  Light-House  Board,  adopted  a 
very  ingenious  plan  for  an  automatic  fog-signal,  which  consisted  in  tak- 
ing advantage  of  a  conical  opening  in  the  coast,  generally  designated  a 
blow-hole.  On  the  apex  of  this  hole  he  erected  a  chimney  which  termi- 
nated in  a  tube  surmounted  by  a  locomotive-whistle.  By  this  arrange- 
ment a  loud  sound  was  produced  as  often  as  a  wave  entered  the  mouth 
of  the  indentation.  The  penetrating  power  of  the  sound  from  this 
arrangement  would  not  be  great  if  it  depended  merely  on  the  hydro- 
static pressure  of  the  wave,  since  this,  under  favorable  circumstances, 
would  not  be  more  than  that  of  a  column  of  water  20  feot  high,  giving 
a  pressure  of  about  10  pounds  to  the  square  inch.  The  effect  however 
of  the  percussion  might  add  considerably  to  this,  though  the  latter 
would  be  confined  in  effect  to  a  single  instant.  In  regard  to  the  prac- 
tical result  from  this  arrangement,  which  was  continued  in  operation 
for  several  years,  it  was  found  not  to  obviate  the  necessity  of  produc- 
ing sounds  of  greater  power.  It  is  however  founded  on  an  ingenious 
idea,  and  may  be  susceptible  of  application  in  other  cases. 


462  RESEARCHES  IN  SOUND. 

EXPERIMENTS  BY  PROFESSOR  ALEXANDER,  IN  1855. 

The  Light- House  Board  was  not  content  with  the  employment  alone 
of  the  fog-signals  in  ordinary  use,  but  directed  a  series  of  experiments 
in  order  to  improve  this  branch  of  its  service.  For  this  purpose  the 
board  employed  Prof.  J.  H.  Alexander,  of  Baltimore,  who  made  a 
report  on  the  subject,  which  was  published  among  the  documents.  The 
investigations  of  Professor  Alexander  related  especially  to  the  use  of 
the  locomotive  steam- whistle  as  a  fog-signal,  and  in  his  report  he  details 
the  results  of  a  series  of  experiments  in  regard  to  the  nature  and  adjust- 
ment of  the  whistle,  the  quantity  of  steam  necessary  to  actuate  it,  with 
suggestions  as  to  its  general  economy  and  management.  He  found, 
what  has  since  been  fully  shown,  that  the  power  of  the  sound  depends 
upon  the  pressure  of  the  steam  in  the  boiler,  and  the  pitch  upon  the 
distance  between  the  circular  orifice  through  which  the  steam  issues, 
and  the  edge  of  the  bell.  He  appears  however  to  be  under  an  erro- 
neous impression  that  the  sound  is  produced  by  the  vibrations  of  the 
metal  of  the  goblet  or  bell,  while  in  fact  this  latter  portion  of  the  appa- 
ratus is  a  resounding  cavity,  which,  as  I  have  shown  in  subsequent 
experiments,  may  be  constructed  of  wood  as  well  as  of  brass,  in  order 
to  produce  the  same  effect.  Mr.  Alexander  also  mentions  the  efiect  of 
the  wind  in  diminishing  the  penetrating  power  of  sound  when  in  an 
adverse  direction,  either  directly  or  approximately.  He  also  recom- 
mends the  adoption  of  an  automatic  pump  to  supply  the  boilers  with 
water,  and  also  to  open  and  shut  the  valves  at  the  proper  intervals  for 
blowing  the  whistle.  He  states  that  the  location  of  a  sound  can  be 
determined  more  precisely  in  the  case  of  loud,  high  sounds  than  in  that 
of  feebler  or  lower  ones.  On  this  point  I  am  not  prepared  to  concur 
with  him  in  experiments  of  my  own.  In  all  cases  however  loud  sounds 
are  more  desirable  than  feebler  ones,  in  order  that  they  may  be  heard 
at  a  greater  distance  above  the  noise  of  the  surf  and  that  of  the  wind 
as  it  passes  through  the  spars  and  rigging  of  vessels. 

The  board  however  at  this  time  were  not  prepared  to  adopt  these 
suggestions,  and  an  unsuccessful  attempt  to  use  a  steam-boiler,  rendered 
abortive  by  the  incapacity  of  the  keeper  to  give  it  proper  attendance, 
discouraged  for  a  time  efforts  in  this  line. 

Previous  to  the  investigations  of  Mr.  Alexander,  at  the  expense  of  the 
Light  Hv>use  Board,  Mr.  Daboll,  of  New  London,  had  for  several  years 
been  experimenting  on  his  own  account  with  reference  to  a  fog-signal. 
His  plan  consisted  in  employing  a  reed  trumpet,  constructed  after  the 
manner  of  a  clarionet,  and  sounded  by  means  of  air  condensed  in  a  res- 
ervoir, the  condensation  being  produced  by  horse-power  operating 
through  suitable  machinery.  Although  the  sound  of  this  was  more  pene- 
trating than  that  of  bells,  still  the  expense  and  inconvenience  of  the 
maintenance  of  a  horse,  together  with  the  cost  of  machinery,  prevented 
its  adoption.  Mr.  Daboll  however  after  this  presented  to  the  board  a 


RESEARCHES    IN    SOUND.  463 

modification  of  his  invention,  in  which  a  hot-air  engine  of  Ericsson's 
patent  was  substituted  as  the  motive-power,  instead  of  the  horse ;  and 
the  writer  ot  this  report,  as  chairman  of  the  committee  on  experiments 
in  behalf  of  the  board,  examined  this  invention  and  reported  in  favor  ot 
its  adoption.  The  other  members  of  the  committee  made  an  unfavorable 
report,  on  the  ground  that  fog-signals  were  of  little  importance,  since  the 
mariner  should  know  his  place  by  the  character  of  his  soundings  in  all 
places  where  accurate  surveys  had  been  made,  or  should  not  venture 
near  the  coast  until  the  fog  was  dissipated.  The  board  however  estab- 
lished Daboll  trumpets  at  different  stations,  which  have  been  in  constant 
use  up  to  the  present  time. 


PAET  I.— INVESTIGATIONS  FKOM  1865  TO  1872.* 
EXPERIMENTS  NEAR  NEW  HAVEN,  IN  1865. 

The  subject  of  sound,  in  connection  with  fog-signals,  still  continued  to 
occupy  the  attention  of  the  board,  and  a  series  of  investigations  was 
made  in  October,  1865,  at  the  light-house  near  New  Haven,  under  the 
direction  of  the  writer  of  this  report,  in  connection  with  Commodore,  now 
Admiral,  Powell,  inspector,  and  Mr.  Ledeile,  acting  engineer  d>f  the  third 
district. 

The  principal  object  was  to  compare  the  sound  of  bells,  of  steam- 
whistles,  and  other  instruments,  and  the  effect  of  reflectors,  and  also  the 
operation  of  different  hot-air  engines.  For  this  purpose  the  committee 
was  furnished  with  two  small  sailing-vessels.  As  these  were  very  imper- 
fectly applicable,  since  they  could  not  be  moved  without  wind,  the  writer 
of  the  report  devised  an  instrument  denominated  an  il  artificial  ear,"  by 
which  the  relative  penetrating  power  of  different  sounding  bodies  could 
be  determined  and  expressed  in  numbers  by  the  removal  of  the  observer 
to  a  comparatively  short  distance  from  the  point  of  origin  of  the  sound. 
This  instrument  consisted  of  a  conical  horn,  made  of  ordinary  tinned 
sheet  iron,  the  axis  of  which  was  about  4  feet  in  length,  the  diameter  of 
the  larger  end  9  inches,  and  tapering  gradually  to  If  of  an  inch  at  the 
smaller  end.  The  axis  of  this  horn  was  bent  at  the  smaller  end  in  a 
gentle  curve,  until  the  plane  of  the  section  of  the  smaller  end  was  at 
light  angles  to  the  perpendicular  section  of  the  larger  end,  so  that  when 
the  axis  of  the  trumpet  was  held  horizontally  and  the  larger  section 
vertically,  then  the  section  of  the  smaller  end  would  be  horizontal. 
Across  the  smaller  end  a  thin  membrane  of  gold  beater's  skin  was  slightly 
stretched  and  secured  by  a  thread.  On  this  membrane  fine  sand  was 
strewn.  To  protect  the  latter  from  disturbance  by  the  wind,  it  was  sur- 
rounded by  a  cylinder  of  glass,  cut  from  a  lamp-chimney,  the  upper  end 
of  which  was  covered  with  a  plate  of  glass ;  and,  in  the  improved  condi 

*  From  the  Report  of  the  Light-House  Board,  for  1874, 


464  RESEARCHES    IN    SOUND. 

tion  of  the  instrument,  with  a  magnifying  lens,  with  which  to  observe 
more  minutely  the  motions  of  the  sand.  To  use  this  instrument  in  com- 
paring the  relative  penetrating  power  of  souud  from  different  sources, 
as  for  example  from  two  bells,  the  axis  bring  held  horizontal,  the  mouth 
was  turned  toward  one  of  the  bells,  and  the  effect  causing  agitation  of 
the  sand,  was  noted.  The  instrument  was  then  removed  to  a  station  a 
little  further  from  the  bell,  and  the  effect  again  noted,  the  distance  being 
increased,  step  by  step,  until  no  motion  in  the  sand  could  be  observed 
through  the  lens.  This  distance,  being  measured  in  feet  or  yards,  gave 
the  number  indicating  the  penetrating  power  of  the  instrument  under 
trial.  The  same  experiment  was  immediately  repeated,  under  the  same 
conditions  of  temperature,  air,  wind,  &c.,  with  the  other  sounding  appa- 
ratus, and  the  relative  number  of  yards  indicating  the  distance,  taken 
as  the  penetrating  powers  of  the  two  instruments.  It  should  be  observed, 
in  the  use  of  this  instrument,  that  it  is  intended  merely  to  concentrate 
the  rays  of  sound,  and  not  to  act  as  a  resounding  cavity ;  since  in  that 
case  the  sound,  in  unison  with  the  resounding  note,  would  produce  effect 
at  a  greater  distance  than  one  in  discord. 

The  indications  of  this  instrument  were  compared  with  the  results  ob- 
tained by  the  ear  in  the  use  of  the  two  vessels,  and  in  all  cases  were  in 
exact  accordance ;  and  it  was  accordingly  used  in  the  following  investi- 
gations, and  has  been  found  of  great  service  in  all  subsequent  experi- 
ments on  the  penetration  of  sound. 

The  only  precaution  in  using  it  is  that  the  membrane  shall  not  be  of 
such  tension  as  to  vibrate  in  unison  with  a  single  sound  or  its  octaves ; 
or,  in  other  words,  that  the  instrument  must  be  so  adjusted  by  varying 
the  length  of  the  axis  or  the  tension  of  ihe  membrane  that  it  shall  be  in 
discordance  with  the  sounds  to  be  measured,  and  only  act  as  a  condenser 
of  the  sonorous  waves. 

The  first  experiments  made  were  with  regard  to  the  influence  of  re- 
flectors. For  this  purpose  a  concave  wooden  reflector  had  been  prepared, 
consisting  of  the  segment  of  a  sphere  of  16  feet  radius,  and  covered  with 
plaster,  exposing  a  surface  of  64  square  feet.  In  the  focus  of  this,  by 
means  of  a  temporary  railway,  a  bell  or  whistle  could  be  readily  intro- 
duced or  withdrawn.  The  center  of  the  mouth  of  the  bell  was  placed  in 
the  horizontal  axis  of  the  reflector.  This  arrangement  being  completed, 
the  sound  of  the  bell,  with  and  without  the  reflector  behind  it,  was  alter- 
nately observed.  Within  the  distance  of  about  500  yards  the  effect  was 
evidently  increased,  as  indicated  by  the  motion  of  the  sand  on  the  mem- 
brane, but  beyond  this  the  difference  was  less  and  less  perceptible,  and 
at  the  limit  of  audibility  the  addition  of  the  reflector  appeared  to  us  en- 
tirely imperceptible.  This  result  was  corroborated  by  subsequent  ex- 
periments in  which  a  whistle  was  heard  nearly  as  well  in  the  rear  of  a 
reflector  as  before  it.  It  would  appear  from  these  results  that  while 
feeble  sounds,  at  small  distances,  are  reflected  as  rays  of  light  are,  waves 


RESEARCHES   IN   SOUND.  465 

of  powerful  sound  spread  laterally,  and  even  when  projected  from  the 
mouth  of  a  trumpet,  tend  at  a  great  distance  to  embrace  the  whole  circle 
of  the  horizon. 

Upon  this  and  all  the  subsequent  experiments,  as  it  will  appear,  the 
principle  of  reflection  as  a  means  of  re-enforcing  sound  is  but  slightly  ap- 
plicable to  fog-signals.  It  is  evident  however  that  the  effect  will  be  some- 
what increased  by  augmenting  the  size  of  the  reflector,  and  by  more  com- 
pletely inclosing  the  source  of  sound  in  a  conical  or  pyramidal  reflector. 

Another  series  of  experiments  was  made  to  ascertain  whether  the  pen- 
etration of  the  sound  was  greater  in  the  direction  of  the  axis  of  the  bells, 
or  at  right  angles  to  the  axis ;  or,  in  other  words,  whether  the  sound  was 
louder  in  front  of  the  mouth  of  a  bell  or  of  its  rim.  The  result  of  this 
experiment  was  considered  of  importance,  since,  in  one  of  the  light- 
houses, a  bell  has  been  placed  with  the  plane  of  its  mouth  at  right  angles 
to  the  horizon,  instead  of  being  placed,  as  usual,  parallel  to  the  same. 
The  effect  on  the  sound  in  these  two  positions  was  similar  to  that  pro- 
duced by  the  bell  with  a  reflector,  the  noise  being  greater  at  a  short 
distance  with  the  mouth  toward  the  observer  than  when  the  rim  was  in 
the  plane  of  the  ear.  At  a  distance  however,  the  difference  between 
the  two  sounds  was  imperceptible.  In  practice  therefore  it  is  of  very 
little  importance  whether  the  axis  of  the  bell  is  perpendicular  or  parallel 
to  the  horizon. 

The  first  fog-signal  examined  in  this  series  of  experiments  was  a  double 
whistle,  improperly  called  a  steam-gong,  designed  principally  for  a  fire- 
alarm  and  for  signals  for  the  comruencejnent  of  working-hours  in  large 
manufacturing  establishments.  It  consisted  of  two  bells  of  the  ordinary 
steam-whistle  on  the  same  hollow  axis,  mouth  to  mouth,  with  a  flat,  hol- 
low cylinder  between  them,  through  the  upper  and  lower  surface  of  which 
the  circular  sheets  of  steam  issue,  the  vibration  of  which  produces  the 
sound.  In  the  instrument  under  examination,  the  upper  bell  was  20 
inches  in  length  of  axis,  and  12  inches  in  diameter,  and  the  lower- whistle 
was  of  the  same  diameter,  with  a  length  of  axis  of  14  inches.  The  note 
of  the  shorter  bell  was  a  fifth  above  that  of  the  longer.  This  arrangement 
gave  a  melodious  sound,  unlike  that  of  the  ordinary  locomotive- whistle, 
and  on  that  account  had  a  peculiar  merit.  The  sound  was  also  very 
loud,  and,  according  to  testimony,  had  been  heard  under  favorable 
circumstances  more  than  twenty  miles.  It  however  required  a  large 
quantity  of  steam  to  give  it  its  full  effect,  and  the  only  means  to  obtain 
an  approximate  idea  as  to  this  quantity  was  that  afforded  by  observing 
its  action  on  a  boiler  of  a  woolen  manufactory  near  Newport.  It  was 
here  blown  with  a  pressure  of  at  least  75  pounds.  From  theoretical  con- 
siderations however,  it  might  be  inferred  that  its  maximum  penetrating 
power  would  be  not  greater  than  that  of  a  single  whistle  using  the  same 
amount  of  steam,  and  this  theoretical  inference  was  borne  out  by  the 
subsequent  experiments  of  General  Duane.  But  from  th«  strikingly 
S.  Mis.  59 — r-30 


466  RESEARCHES   IN   SOUND. 

distinctive  character  of  its  tone  it  has,  in  our  opinion,  an  advantage  over 
a  single  whistle  expending  an  equal  quantity  of  steam. 

The  fact  that  the  vibration  of  the  metal  of  the  bell  had  no  practical 
effect  on  the  penetrating  power  of  the  sound  was  proved  quite  conclu- 
sively by  winding  tightly  around  each  bell,  over  its  whole  length,  a  thick 
cord,  which  would  effectually  stop  all  vibration.  ^The  penetration  of  the 
sound  produced  under  this  condition  was  the  same  as  that  with  the  bells 
free.  It  is  true,  the  latter  produces  a  difference  in  the  quality  of  the 
tone,  such  as  that  which  is  observed  in  a  brass  instrument  and  that  of 
one  of  wood  or  ivory.  The  inventor  was  not  aware  that  the  sound  pro- 
duced was  from  the  resonance  of  the  air  within  the  bell,  and  not  from 
the  metal  of  the  bell  itself,  and  had  obtained  a  patent,  not  only  for  the 
invention  of  the  double  whistle,  but  also  for  the  special  compound  of 
metal  of  which  it  was  composed. 

Another  apparatus  proposed  to  be  used  as  a  fog-signal  was  presented 
for  examination  by  the  Marine  Signal  Company,  of  Wallingford,  Conn. 
It  consisted  of  a  curved  tube  of  copper  nearly  in  the  form  of  the  letter 
C,  and  was  supported  on  an  axis  passing  through  the  center  of  the 
figure.  An  ordinary  bell- whistle  was  attached  to  each  extremity  of  the 
tube,  the  instrument  being  placed  in  a  vertical  position  and  partially 
filled  with  water,  then  made  to  oscillate  on  its  center  of  support.  By 
this  means  the  air  was  drawn  in  at  one  end  and  forced  out  through  the 
whistle  at  the  other.  The  motion  being  reversed  the  air  was  drawn  in 
at  the  end  through  which  it  had  just  made  its  exit  and  forced  out  through 
the  whistle  at  the  other.  By  rocking  the  instrument,  either  by  hand  or 
by  the  motion  of  the  vessel,  a  continued  sound  could  be  produced.  The 
motive-power  in  the  former  case  was  muscular  energy,  and  the  experi- 
ments which  were  made  at  this  time,  as  well  as  all  that  have  been  made 
subsequently,  conclusively  prove  that  the  penetrating  power  of  the 
sound  for  practical  use  as  a  fog-signal  depends  upon  the  intensity  of  the 
motive-energy  employed.  No  instrument  operated  through  levers  and 
pumps  by  hand-power  is  sufficient  for  the  purpose. 

One  of  these  instruments  with  two  4-inch  whistles  gave  a  sound,  as 
indicated  by  the  artificial  ear,  the  power  of  which  was  about  one- tenth 
of  that  of  a  steam-trumpet.  It  was  supposed  however  that  this  in- 
strument would  be  applicable  for  light-ships  ;  and  that  if  extended  en- 
tirely across  the  vessel,  and  armed  with  whistles  of  large  size,  it  would 
be  operated  by  the  rolling  of  the  vessel,  and  thus  serve  to  give  warning 
in  time  of  thick  weather.  But  as  it  frequently  happens  that  fog  exists 
during  a  calm,  this  invention  could  not  be  relied  upon  to  give  warning 
in  all  cases  of  danger.  Besides  this,  the  ordinary  roll  of  a  ship  is  not 
sufficient  to  produce  a  hydrostatic  pressure  of  more  than  five  or  six 
pounds  to  the  square  inch,  which  is  insufficient  to  give  an  effective 
sound.  It  has  however  been  proposed  to  increase  the  power  by  using 
quicksilver  instead  of  water;  but,  besides  the  first  cost  of  this  material, 
and  the  constant  loss  by  leakage  and  oxidation,  the  tendency  to  affect 


RESEARCHES    IN    SOUND.  467 

the  health  of  the  crew  is  an  objection  to  the  introduction  of  this  modi- 
fication of  the  apparatus  into  light-ships. 

The  other  instruments  which  were  subjected  to  trial  were  an  ordinary 
steam-whistle  and  a  Daboll  trumpet.  The  bell  of  the  whistle  was  6 
inches  in  diameter,  9  inches  in  height,  and  received  the  sheet  of  steam 
through  an  opening  of  one-thirtieth  of  an  inch  in  width ;  was  worked 
by  a  pressure  of  condensed  air  of  from  20  to  35  pounds  per  square  inch, 
and  blown  once  in  a  minute  for  about  five  seconds.  The  air  was  con- 
densed by  a  Eoper  engine  of  one-horse  power.  The  penetrating  power 
of  the  sound  was  increased  by  an  increase  in  the  pressure  of  the  air, 
and  also  the  pitch.  The  tone  however  of  the  instrument  was  lowered 
by  increasing  the  distance  between  the  orifice  through  which  the  circular 
sheet  of  air  issued  at  the  lower  rim  of  the  bell  or  resounding  cavity. 
To  prove  conclusively  that  the  bell  performs  the  part  of  a  mere  resound- 
ing cavity,  a  wooden  one,  on  a  subsequent  occasion,  was  substituted  for 
that  of  metal  without  a  change  in  the  loudness  or  the  pitch  of  the  sound. 

The  penetrating  power  of  the  whistle  was  compared  with  a  Daboll 
trumpet,  actuated  by  an  Ericsson  engine  of  about  the  same  power ;  the 
reservoir  for  the  condensed  air  of  each  machine  was  furnished  with  a 
pressure-gauge,  and  by  knowing  the  capacity  of  the  condensing  pumps 
and  the  number  of  strokes  required  to  produce  the  pressure,  the  relative 
amount  of  power  was  determined.  The  result  was  that  the  penetrating 
power  of  the  trumpet  was  nearly  double  that  of  the  whistle,  and  that  an 
equal  effect  was  produced  afc  the  same  distance  by  about  one-fourth  of 
the  power  expended  in  the  case  of  the  latter.  It  must  be  recollected 
however,  that  the  whistle  sends  sonorous  waves  of  equal  intensity  in 
every  direction,  while  the  greatest  power  of  the  trumpet  is  in  the  direc- 
tion of  its  axis.  This  difference  however  is  lessened  on  account  of  the 
spreading  of  the  sound  to  which  we  have  before  alluded.*  The  whistle 
was  blown,  as  we  have  said,  with  a  pressure  of  from  20  to  35  pounds, 
while  the  trumpet  was  sounded  with  a  pressure  of  from  12  to  15  pounds. 
In  the  case  of  the  whistle,  the  pressure  in  the  reservoir  may  be  indefi- 
nitely increased  with  an  increase  in  the  penetrating  power  of  the  sound 
produced,  while  in  the  case  of  the  trumpet  a  pressure  greater  than  a 
given  amount  entirely  stops  the  blast  by  preventing  the  recoil  of  the 
vibrating  tongue ;  this  being  made  of  steel,  in  the  larger  instruments 
2£  inches  wide  and  8  inches  long,  would  receive  a  pressure  of  steam,  at 
only  10  pounds  to  the  square  inch,  of  200  pounds,  tending  to  press  it 
into  the  opening  and  to  prevent  its  recoil,  this  circumstance  limits,  as  it 
were,  the  power  of  a  trumpet  of  given  dimensions.  It  is  however  well 
fitted  to  operate  with  a  hot-air  engine,  and  is  the  least  expensive  in  fuel 
of  any  of  the  instruments  now  employed.  The  whistle  is  the  simpler 
and  easier  of  management,  although  they  both  require  arrangement  of 
machinery  in  order  that  they  may  be  operated  automatically. 

*  It  is  worthy  of  note  however  that  in  the  case  of  a  sound  having  primarily  an  axial 
direction,  the  subsequent  lateral  diffusion  must  result  in  enfeebling  the  whole  sphere 
of  expanding  sound-waves  in  a  more  rapid  ratio  than  the  square  of  the  distance. 


468  RESEARCHES   IN   SOUND. 

It  is  a  matter  of  much  importance  to  obtain  a  hot-air  engine  of  suffi- 
cient power,  and  suitable  for  working  fog-signals  of  all  classes.  This 
will  be  evident  when  we  consider  the  difficulty  in  many  cases  of  obtain- 
ing  fresh  water  for  producing  steam,  and  the  expense  of  the  renewal  of 
the  boilers  in  the  use  of  salt- water,  as  well  as  that  of  the  loss  of  power 
in  frequently  blowing  out  the  latter,  in  addition  to  the  danger  of  the  use 
of  steam  by  unskillful  attendants. 

The  merits  of  the  two  engines  however  under  consideration  could  not 
be  fully  tested  by  the  short  trial  to  which  they  were  subjected  during 
these  experiments.  The  principal  objection  to  the  Ericsson  engine  was 
the  size  of  the  fly-wheel  and  the  weight  of  the  several  parts  of  the  ma- 
chine ;  the  Eoper  engine  was  much  more  compact,  and  appeared  to  work 
with  more  facility,  but  from  the  greater  heat  imparted  to  the  air  the 
packing  was  liable  to  burn  out  and  required  to  be  frequently  renewed. 
Although  at  first  the  impression  of  the  committee  was  in  favor  of  the 
Eoper  engine,  yet  in  subsequent  trials  of  actual  practice  it  was  found 
too  difficult  to  be  kept  in  order  to  be  employed  for  light-house  purposes, 
and  its  use  has  consequently  been  abandoned ;  another  hot-air  engine 
has  been  employed  by  the  board,  the  invention  of  a  Mr.  Wilcox,  which 
has  also  been  discontinued  for  a  similar  reason.  I  was  assured  by  the 
person  last  named,  a  very  ingenious  mechanician,  that  when  the  several 
patents  for  hot-air  engines  expired,  a  much  more  efficient  instrument 
could  be  devised  by  combining  the  best  features  of  each  of  those  now  in 
use. 

For  determining  the  relative  penetrating  power  of  these  instruments, 
the  use  of  two  vessels  had  been  obtained,  with  the  idea  of  observing  the 
sound  simultaneously  in  opposite  directions. 

Unfortunately  however  the  location  which  had  been  chosen  for  these 
experiments  was  of  a  very  unfavorable  character  in  regard  to  the  em- 
ployment of  sailing-vessels  and  the  use  of  the  artificial  ear.  It  was  fully 
open  to  the  ocean  only  in  a  southerly  direction,  navigation  up  the  bay 
to  the  north  being  limited  to  three  and  a  half  miles,  while  on  shore  a 
sufficient  unobstructed  space  could  not  be  obtained  for  the  proper  use 
of  the  artificial  ear.  With  these  obstructions  and  the  necessity  of  beat- 
ing against  the  wind,  thereby  constantly  altering  the  direction  of  the 
vessel,  exact  comparisons  were  not  possible,  yet  the  observations  made 
were  sufficiently  definite  to  warrant  certain  conclusions  from  them  as  to 
the  relative  power  of  the  various  instruments  submitted  to  examination. 

The  following  is  a  synopsis  of  the  observations  on  four  different  days. 
Before  giving  these  however,  it  is  necessary  to  observe  that  at  each 
stroke  of  the  piston  of  the  hot-air  engine  a  loud  sound  was  produced  by 
the  blowing  off  of  the  hot  air  from  the  cylinder,  after  it  has  done  its  work. 
In  the  following  statement  of  results  the  noise  thus  produced  is  called 
the  exhaust.  On  the  first  day  but  one  set  of  observations  was  made,  the 
vessel's  course  being  nearly  in  the  line  of  the  axis  of  the  trumpet.  The 


RESEARCHES    IN    SOUND.  469 

order  of  penetrating  power  was  as  follows:  1,  trumpet ;  2,  exhaust;  3, 
bell;  these  instruments  being  heard  respectively  at  5-J,  3J,  and  2  miles. 
The  whistle  was  not  sounded. 

The  second  day  simultaneous  observations  were  made  from  two  ves- 
sels sailing  nearly  in  opposite  directions.  The  results  of  the  observations 
made  on  the  vessel  sailing  in  a  southerly  direction  were  very  irregular. 
The  trumpet  was  heard  at  3f  miles  and  lost  at  4f  miles  with  the  wind 
slightly  in  favor  of  the  sound,  and  heard  at  6  J  miles  with  the  wind  some- 
what against  the  sound ;  it  was  heard  even  at  7-jj  miles  from  the  mast- 
head, though  inaudible  from  the  deck.  In  all  these  cases  the  position 
of  the  vessel  was  nearly  in  line  with  the  axis  of  the  trumpet. 

The  whistle  and  exhaust  were  heard  at  7J  miles  with  a  feeble  oppos- 
ing wind,  and  lost  at  6J  miles  when  the  force  of  the  wind  became  greater. 

The  order  of  penetration  in  this  series  of  observations  was :  1,  trumpet 
and  gong;  2,  whistle;  3,  exhaust. 

In  the  case  of  the  vessel  sailing  northward,  its  course  being  almost 
directly  against  the  wind  and  in  the  rear  of  the  trumpet,  all  the  sounds 
were  lost  at  less  distances  than  in  the  case  of  the  other  vessel.  The 
observations  showed  very  clearly  the  effect  of  the  wind,  the  bell  at  a 
certain  distance  being  heard  indistinctly  with  a  strong  opposing  wind 
and  more  and  more  plainly  as  the  wind  died  away.  The  trumpet  was 
heard  only  as  far  as  the  whistle,  the  vessel  being  in  the  rear  of  It. 

The  third  day  observations  were  made  from  the  two  vessels,  both 
however  sailing  to  the  south.  From  the  vessel  sailing  at  right  angles 
to  the  direction  of  the  wind  the  order  of  penetration  was :  1,  trumpet ; 
2,  whistle;  3,  exhaust;  4,  bell. 

In  the  case  of  the  other  vessel  the  opposing  effect  of  the  wind  was 
greater,  and  the  sounds  were  heard  to  a  less  distance;  the  order  was: 
1.  trumpet;  2,  whistle;  3,  exhaust;  4,  bell;  5,  rocker. 

On  the  fourth  day  two  trips  were  made  by  the  same  vessel  in  the 
course  of  the  day,  one  being  northward  and  the  other  southward.  In 
the  first  case  the  trumpet  was  lost  at  3J  miles,  the  vessel  being  nearly 
in  its  rear;  in  the  second  case,  the  wind  being  almost  directly  opposed 
to  the  sound,  the  large  bell  was  heard  at  1J  miles,  and  lost  at  J-  of  a 
mile,  probably  due  to  increase  of  the  force  of  the  wind;  the  trumpet  was 
was  lost  at  3^  miles. 

In  all  these  observations,  owing  to  the  unfavorable  conditions  of  the 
locality,  and  the  direction  of  the  wind,  we  were  unable  to  obtain  any 
satisfactory  observations  on  sound  moving  with  the  wind.  In  all  cases 
the  results  were  obtained  from  sounds  moving  nearly  against  the  wind, 
or  at  right  angles  to  it.  From  the  results  of  the  whole  it  appears  that 
the  sound  was  heard  farther  with  a  light  opposing  wind  than  with  a 
stronger  one,  and  that  it  was  heard  farthest  of  all  at  right  angles  to  the 
wind.  From  this  latter  fact,  however,  it  should  not  be  inferred  that  in 
this  case  sound  could  be  heard  farther  at  right  angles  to  the  wind  than 
with  the  wind,  but  that  in  this  direction  the  effect  of  the  wind  was  neu- 


470  EESEARCHES   IN   SOUND. 

tralized.  The  results  also  exhibited,  in  a  striking  manner,  the  diverg- 
ency of  sound  from  the  axis  of  the  trumpet,  the  trumpet  being  heard  in 
the  line  of  its  axis  in  front  at  6  miles  and  behind  at  3,  the  wind  being 
nearly  the  same  in  both  cases. 

All  the  observations  were  repeated  on  land  with  the  artificial  ear  as 
far  as  the  unfavorable  condition  of  the  surface  would  permit.  Although 
the  limit,  as  to  distance,  at  which  the  sand  might  be  moved  was  not  in 
most  cases  observed,  yet  the  relative  degree  of  agitation  at  a  given  dis- 
tance established  clearly  which  was  the  most  powerful  instrument,  the 
result  giving  precisely  the  same  order  of  penetration  of  the  different  in- 
struments as  determined  by  direct  andition. 

During  this  series  of  investigations  an  interesting  fact  was  discovered, 
namely,  a  sound  moving  against  the  wind,  inaudible  to  the  ear  on  the 
deck  of  the  schooner,  was  heard  by  ascending  to  the  mast-head.  This 
remarkable  fact  at  first  suggested  the  idea  that  sound  was  more  readily 
conveyed  by  the  upper  current  of  air  than  the  lower,  and  this  appeared 
to  be  in  accordance  with  the  following  statement  of  Captain  Keeney, 
who  is  commander  of  one  of  the  light-house  vessels,  and  has  been  for  a 
long  time  on  the  banks  of  Newfoundland  in  the  occupation  of  fishing  : 
"When  the  fishermen  in  the  morning  hear  the  sound  of  the  surf  to  the 
leeward,  or  from  a  point  toward  which  the  wind  is  blowing,  they  take 
this  as  an  infallible  indication  that  in  the  course  of  from  one  to  five  hours 
the  wind  will  change  to  the  opposite  direction  from  which  it  is  blowing  at 
the  time.77  The  same  statement  was  made  to  me  by  the  intelligent  keeper 
of  the  fog-signal  at  Block  Island.  In  these  cases  it  would  appear  that 
the  wind  had  already  changed  direction  above,  and  was  thus  transmit- 
ting the  sound  in  an  opposite  direction  to  that  of  the  wind  at  the  surface 
of  the  earth. 

Another  remarkable  fact  bearing  on  this  same  point  is  established  by 
the  observations  of  General  Duane.  At  Cape  Elizabeth,  9  miles  south, 
easterly  from  the  general's  house,  at  Portland,  is  a  fog-signal  consisting 
of  a  whistle  10  inches  in  diameter ;  at  Portland  Head,  about  4  miles  from 
the  same  city,  in  nearly  the  same  direction,  is  a  Daboll  trumpet.  There 
can  be  no  doubt,  says  the  general,  that  those  signals  can  be  heard  much 
better  during  a  heavy  northeast  snow-storm  than  at  any  other  time. 
"As  the  wind  increases  in  force,  the  sound  of  the  nearer  instrument,  the 
trumpet,  diminishes,  but  the  whistle  becomes  more  distinct ;  but  I  have 
never  known  the  wind  to  blow  hard  enough  to  prevent  the  sound  of  the 
latter  from  reaching  this  city."  In  this  case,  the  sound  comes  to  the  city 
in  nearly  direct  opposition  to  the  course  of  the  wind,  and  the  explanation 
which  suggested  itself  to  me  was  that  during  the  continuance  of  the 
storm,  while  the  wind  was  blowing  from  the  northeast  at  the  surface, 
there  was  a  current  of  equal  or  greater  intensity  blowing  in  an  opposite 
direction  above,  by  which  the  sound  was  carried  in  direct  opposition  to 
the  direction  of  the  surface  current.  The  existence  of  such  an  upper 
current  is  in  accordance  with  the  hypothesis  of  the  character  of  a  north- 


RESEARCHES    IN    SOUND.  471 

east  storm,  which  sometimes  rages  for  several  days  at  a  given  point  on 
the  coast  without  being  felt  more  than  a  few  miles  in  the  interior,  the  air 
continuously  flowing  in  below  and  going  out  above.  Indeed,  in  such 
cases  a  break  in  the  lower  clouds  reveals  the  fact  of  the  existence  above 
of  a  rapid  current  in  the  opposite  direction. 

The  full  significance,  however,  of  this  idea  did  not  reveal  itself  to  me 
until  in  searching  the  bibliography  of  sound  I  found  an  account  of  the 
hypothesis  of  Professor  Stokes  in  the  Proceedings  of  the  British  Associa- 
tion for  185G,  *  in  which  the  effect  of  an  upper  current  in  deflecting  the 
wave  of  sound  so  as  to  throw  it  down  upon  the  ear  of  the  auditor,  or 
directing  it  upward  far  above  his  head,  is  fully  explained.  This  subject 
will  be  referred  to  in  the  subsequent  parts  of  the  report,  in  the  attempt 
to  explain  various  abnormal  phenomena  of  sound  which  have  been  ob- 
served during  the  series  of  investigations  connected  with  the  Light- 
House  Board. 

During  these  investigations  an  attempt  was  made  to  ascertain  the 
velocity  of  the  wind  in  an  upper  stratum  as  compared  with  that  in  the 
lower.  The  only  important  result  however,  was  the  fact  that  the  veloc- 
ity of  the  shadow  of  a  cloud  passing  over  the  ground  was  much  greater 
than  that  of  the  air  at  the  surface,  the  velocity  of  the  latter  being  de- 
termined approximately  by  running  a  given  distance  with  such  speed 
that  a  small  flag  was  at  rest  along  the  side  of  its  pole.  While  this 
velocity  was  not  perhaps  greater  than  six  miles  per  hour,  that  of  the 
shadow  of  the  cloud  was  apparently  equal  to  that  of  a  horse  at  full 
speed. 

During  this  and  subsequent  investigations,  inquiries  were  made  in 
regard  to  the  effect  of  fog  upon  sound,  it  being  a  subject  of  considera- 
ble importance  to  ascertain  whether  waves  of  sound,  like  the  rays  of 
light,  are  absorbed  or  stifled  by  fog.  On  this  point,  however,  observers 
disagree.  At  first  sight,  from  the  very  striking  analogy  which  exists  in 
many  respects  between  sound  and  light,  the  opinion  largely  prevails 
that  sound  is  impeded  by  fog;  although  observers  who  have  not  been 
influenced  by  this  analogy  have,  in  many  instances,  adopted  the  opposite 
opinion,  that  sound  is  better  heard  during  a  fog  than  in  clear  weather. 
For  instance,  the  Kev.  Peter  Ferguson,  of  Massachusetts,  informs  me 
that  from  his  own  observations,  sound  is  conveyed  farther  in  a  fog  than 
in  a  clear  air.  He  founds  this  opinion  on  observations  which  he  has 
made  on  the  sound  of  locomotives  of  several  railways  in  passing  over 
bridges  at  a  distance.  Unfortunately,  the  question  is  a  difficult  one  to 
settle,  since  the  effect  of  the  wind,  in  order  to  arrive  at  a  true  result, 
must  be  carefully  eliminated.  Captain  Keeney,  who  has  previously 
been  mentioned,  related  the  following  occurrence,  in  the  first  part  of 
which  he  was  led  to  suppose  that  fog  had  a  very  marked  influence  in 
deadening  sound,  though  in  a  subsequent  part  he  came  to  an  opposite 
conclusion  :  He  was  sailing  during  a  dense  fog,  with  a  slight  wind  bear- 

*  Report  of  British  Association,  1856 ;  Abstracts,  p.  22. 


472  RESEARCHES   IN   SOUND. 

ing  him  toward  a  light-vessel,  the  locality  of  which  he  expected  to  find 
by  means  of  the  fog-signal.  He  kept  on  his  course  until  he  thought 
himself  very  near  the  ship,  without  hearing  the  stroke  of  the  bell.  He 
then  anchored  for  the  night,  and  found  himself  next  morning  within  a 
short  distance  of  the  light- vessel,  but  still  heard  no  sound,  although  he 
was  assured  when  he  got  to  it  that  the  bell  had  been  ringing  all  night. 
He  then  passed  on  in  the  same  direction  in  which  he  had  previously 
sailed,  leaving  the  light- vessel  behind,  and  constantly  heard  the  bell  for 
a  distance  of  several  miles,  the  density  of  the  fog  not  perceptibly  dimin- 
ishing. In  this  case  it  is  evident  that  the  deadening  of  the  sound  was 
not  due  to  the  fog,  but,  as  we  shall  hereafter  see,  in  all  probability  to  the 
combined  action  of  the  upper  and  the  lower  currents  of  air. 

On  returning  to  Washington  the  writer  took  advantage  of  the  occur- 
rence of  a  fog  to  make  an  experiment  as  to  the  penetration  of  the  sound 
of  a  small  bell  rung  by  clock-work,  the  apparatus  being  the  part  of  a 
moderator-lamp  intended  to  give  warning  to  the  keepers  when  the  sup- 
ply of  oil  ceased.  The  result  of  the  experiment  was  contrary  to  the  sup- 
position of  absorption  of  the  sound  tiy  the  fog,  but  the  change  in  the 
condition  of  the  atmosphere  as  to  temperature  and  the  motion  of  the 
air,  before  the  experiment  could  be  repeated  in  clear  weather,  rendered 
the  result  not  entirely  satisfactory. 

EXPERIMENTS  AT  SANDY  HOOK  IN  18G7. 

The  next  series  of  experiments  was  made  from  October  10  to  October 
18,  1867,  under  the  direction  of  the  writer  of  this  report,  in  connection 
with  General  Poe,  engineer-secretary  of  the  Light-House  Board,  Com- 
modore (now  Admiral)  Case,  then  inspector  of  the  third  light-house  dis- 
district,  and  Mr.  Lederle,  acting  engineer  of  the  same  district. 

The  principal  object  of  these  investigations  was  to  compare  different 
instruments,  and  to  ascertain  the  improvements  which  had  been  made 
in  them  since  the  date  of  the  last  investigations,  especially  the  exami- 
nation of  a  new  fog-signal  called  the  siren,  and  the  comparison  of  it  with 
the  Daboll  trumpet,  although  other  investigations  were  made  relative  to 
the  general  subject  of  sound  in  relation  to  fog-signals.  The  locality 
chosen  was  Sandy  Hook,  a  narrow  peninsula  projecting  northward, 
about  five  miles  into  the  middle  of  the  Lower  Bay  of  New  York,  and  al- 
most at  right  angles  to  its  coast,  having  a  width  of  about  half  a  mile. 
Near  the  northern  point  on  the  east  shore  a  temporary  building  was 
erected  for  the  shelter  of  the  engines  and  other  instruments. 

The  comparisons  in  regard  to  penetrating  power  were  made  by  the 
use  of  the  artificial  ear,  heretofore  described,  by  carrying  this  off  a 
measured  distance  until  the  sand  ceased  to  move.  This  operation  was 
much  facilitated  by  previous  surveys  of  members  of  the  Engineer 
Corps,  who  had  staked  off  a  straight  line  parallel  with  the  shore,  and 
accurately  divided  it  into  equal  distances  of  100  feet. 

On  account  of  the  character  of  the  deep  and  loose  sand,  walking 


RESEARCHES    IN   SOUND.  473 

along  this  distance  was  exceedingly  difficult,  and,  to  obviate  this,  a  car- 
riage with  broad  wheels,  drawn  by  two  horses,  was  employed.  An 
awning  over  this  vehicle  protected  the  observer  from  the  sun,  and  ena- 
bled him,  without  fatigue  and  at  his  ease,  to  note  the  agitations  of 
sand  on  the  drum  of  the  artificial  ear,  the  mouth  of  which  was  directed 
from  the  rear  of  the  carriage  toward  the  sounding  instrument. 

For  these  and  other  facilities  we  were  indebted  to  General  Humphreys, 
Chief  of  the  Engineer  Bureau,  who  gave  orders  to  the  officer  in  charge 
of  the  military  works  at  Sandy  Hook  to  afford  us  every  aid  in  his  power 
in  carrying  on  the  investigation. 

The  instruments  employed  were — 

1st.  A  first-class  Daboll  trumpet  (the  patent  for  which — since  the 
death  of  Mr.  Daboll,  is  owned  by  Mr.  James  A.  Eobinson,)  operated  by 
an  Ericsson  hot-air  engine.  It  carried  a  steel  reed  10  inches  long,  2|  inches 
wide,  and  J  inch  in  thickness  at  the  vibrating  end,  but  increasing  gradu- 
ally to  an  inch  at  the  larger  extremity.  This  was  attached  to  a  large 
vertical  trumpet  curved  at  the  upper  end  into  a  horizontal  direction  and 
furnished  with  an  automatic  arrangement  for  producing  an  oscillation 
of  the  instrument  of  about  GO0  in  the  arc  of  the  horizon.  Its  entire 
length,  including  the  curvature,  was  17  feet.  It  was  3J  inches  at  the 
smaller  end  and  had  a  flaring  mouth  38  inches  in  diameter.  The  engine 
had  a  cylinder  32  inches  in  diameter,  with  an  air-chamber  of  4J  feet  in 
diameter  and  6  feet  long,  and  was  able  to  furnish  continually  a  five-second 
blast  every  minute  at  a  pressure  of  from  15  to  30  pounds. 

2d.  A  siren,  originally  invented  by  Cagniard  de  Latour,  and  well 
known  to  the  physicist  as  a  means  of  comparing  sounds  and  measuring 
the  number  of  vibrations  in  different  musical  notes.  Under  the  direc- 
tion of  the  Light-House  Board,  Mr.  Brown,  of  New  York,  had  made  a 
series  of  experiments  on  this  instrument  ill  reference  to  its  adoption  as 
a  fog-signal,  and  these  experiments  have  been  eminently  successful. 
The  instrument  as  it  now  exists  differs  in  two  essential  particulars 
from  the  original  invention  of  Latour:  1st,  it  is  connected  with  a 
trumpet  in  which  it  supplies  the  place  of  the  reed  in  producing  the  agi- 
tation of  the  air  necessary  to  the  generation  of  the  sound ;  and  2d,  the 
revolving  disk,  which  opens  and  shuts  the  orifices  producing  the  blasts, 
is  driven  not  by  the  blast  itself  impinging  on  oblique  openings,  as  in 
the  original  instrument,  but  by  a  small  engine  connected  with  the  feed- 
pump of  the  boiler. 

The  general  character  of  the  instrument  may  be  understood  from  the 
following  description:  Suppose  a  drum  of  short  axis,  into  one  head  of 
which  is  inserted  a  steam-pipe  connected  with  a  locomotive-boiler,  while 
the  other  end  has  in  it  a  triangular  orifice,  through  which  the  steam  is 
at  brief  intervals  allowed  to  project  itself.  Ira  mediately  before  this  head, 
and  in  close  contact  with  it,  is  a  revolving  disk,  in  which  are  eight 
orifices.  By  this  arrangement,  at  every  complete  revolution  of  the  disk, 
the  orifice  in  the  head  of  the  drum  is  opened  and  shut  eight  times  in 


474  EESEARCHES    IN   SOUND. 

succession,  thus  producing  a  rapid  series  of  impulses  of  steam  against  the 
air  into  the  smaller  orifice  of  the  trumpet  placed  immediately  in  front  of 
the  revolving  disk.  These  impulses  are  of  such  intensity  and  rapidity 
as  to  produce  a  sound  unrivalled  in  magnitude  and  penetrating  power 
by  that  of  any  other  instrument  yet  devised. 

The  siren  was  operated  by  an  upright  cylindrical  tubular  boiler,  with 
a  pressure  of  from  50  to  100  pounds  on  the  square  inch.  For  this  form 
of,  boiler  has  been  subsequently  substituted  an  ordinary  horizontal 
locomotive-boiler  with  a  small  engine  attached  for  feeding  it  and  for 
rotating  the  disk,  the  latter  being  effected  by  means  of  a  band  passing 
over  pulleys  of  suitable  relative  dimensions. 

3d.  A  steam-whistle  8  inches  in  diameter.  Through  some  misunder- 
standing a  series  of  whistles  of  different  diameters  was  not  furnished 
as  was  intended. 

The  first  experiments  to  be  noted  were  those  in  regard  to  the  com- 
parison of  penetrating  power  of  the  siren  and  the  whistle,  the  fitting  up 
of  the  Daboll  trumpet  not  having  been  completed.  The  principal  object 
of  this  however  was  to  test  again  the  truthfulness  of  the  indications  of 
the  artificial  ear  in  comparison  with  those  of  the  natural  ear. 

An  experiment  was  made  both  by  means  of  the  artificial  ear  on  land 
and  by  actually  going  off  on  the  ocean  in  a  steamer  until  the  sounds  be- 
came inaudible  to  the  natural  ear.  By  the  latter  method  the  two  sounds 
ceased  to  be  heard  at  the  distances  of  six,  and  twelve  and  a  half  miles, 
respectively.  The  indications  of  the  artifical  ear  gave  a  similar  result, 
the  distance  at  which  the  sand  ceased  to  move  in  one  case  being  double 
that  of  the  other.  In  both  cases  the  conditions  of  wind  and  weather 
were  apparently  the  same.  In  the  case  of  the  steamer  the  distance  was 
estimated  by  noting  the  interval  of  time  between  the  flash  of  steam  and 
the  perception  of  the  sound. 

Comparison  of  the  Daboll  trumpet  and  the  siren. — The  pressure  of  the 
hot  air  in  the  reservoir  of  the  hot-air  engine  of  the  trumpet  was  about 
20  pounds,  and  that  of  the  steam  in  the  boiler  of  the  siren  about  75 
pounds.  These  pressures  are  however  not  considered  of  importance  in 
these  experiments,  since  the  object  was  not  so  much  to  determine  the 
relative  amount  of  motive  power  employed  as  the  amount  of  penetrat- 
ing power  produced  by  these  two  instruments,  each  being  one  of  the 
first  of  its  class. 

1.  At  distance  50  the  trumpet  produced  a  decided  motion  of  the  sand, 
while  the  siren  gave  a  similar  result  at  distance  58.     The  two  observa- 
tions being  made  within  ten  minutes  of  each  other,  it  may  be  assumed 
that  the  condition  of  the  wind  was  the  same  in  the  two  cases,  and  hence 
the  numbers  above  given  may  be  taken  as  the  relative  penetrating  power 
of  the  two  instruments. 

2.  Another  series  of  experiments  was  instituted  to  determine  whether  a 
high  or  a  low  note  gave  the  greatest  penetration.    For  this  purpose  the 


RESEARCHES   IN   SOUND.  475 

siren  was  sounded  with  different  velocities  of  rotation  of  the  perforated 
disk,  the  pressure  of  steam  remaining  at  90  pounds  per  square  inch. 
The  effect  upon  the  artificial  ear  in  causing  greater  or  less  agitation  of 
sand  was  taken  as  the  indication  of  the  penetrating  power  of  the  differ- 
ent tones.  The  number  of  revolutions  of  the  disk  in  a  given  time  was 
determined  by  a  counting  apparatus,  consisting  of  a  train  of  wheels  and 
a  series  of  dials  showing  tens,  hundreds,  and  thousands  of  revolutions ; 
this  was  temporarily  attached  to  the  projecting  end  of  the  spindle  of 
the  revolving  disk  by  pushing  the  projecting  axis  of  the  instrument  into 
a  hole  in  the  end  of  the  spindle. 

From  the  whole  of  this  series  of  experiments  it  appeared  that  a  revolu- 
tion which  gave  400  impulses  in  a  second  was  the  best  with  the  siren 
when  furnished  with  a  trumpet.  On  reflection  however  it  was  con- 
cluded that  this  result  might  not  be  entirely  due  to  the  pitch,  but  in 
part  to  the  perfect  unison  of  that  number  of  impulses  of  the  siren  with 
the  natural  tone  of  the  trumpet.  To  obviate  this  complication,  a  series 
of  experiments  was  next  day  made  on  the  penetration  of  different  pitches 
with  the  siren  alone,  the  trumpet  being  removed.  The  result  was  as 
follows : 

The  siren  was  sounded  at  five  different  pitches,  the  artificial  ear  be- 
ing at  such  a  distance  as  to  be  near  the  limit  of  disturbance  by  the 
sound.  In  this  condition  the  lowest  pitch  gave  no  motion  of  sand.  A 
little  higher,  slight  motion  of  sand.  Still  higher,  considerable  motion 
of  sand ;  and  with  a  higher  pitch  again,  no  motion  of  sand.  The  best 
result  obtained  was  with  a  revolution  which  gave  3GO  impulses  in  a 
second. 

3.  An  attempt  was  made  to  determine  the  most  effective  pitch  or  tone 
of  the  steam-whistle.  It  was  started  with  what  appeared  to  be  the  funda- 
mental note  of  the  bell,  which  gave  slight  motion  of  sand ;  a  higher 
tone  a  better  motion ;  still  higher,  sand  briskly  agitated;  next,  several 
tones  lower,  no  motion;  higher,  no  motion;  still  higher,  no  motion.  The 
variation  in  the  tone  was  made  by  altering  the  distance  between  the  bell 
and  the  orifice  through  which  the  steam  was  ejected. 

The  result  of  this  experiment  indicated  nothing  of  a  definite  charac- 
ter, other  than  that  with  a  given  pressure  there  is  a  maximum  effect 
produced  when  the  vibrations  of  the  sheet  of  air  issuing  from  the  circu- 
lar orifice  are  in  unison  with  the  natural  vibrations  from  the  cavity  of 
the  bell,  a  condition  which  can  only  be  determined  in  any  case  by  actual 
experiment.  In  practice,  Mr.  Brown  was  enabled  to  produce  the  best 
effect  by  regulating  the  velocity  until  the  trumpet  gave  the  greatest 
penetrating  power,  as  indicated  by  an  artificial  ear  of  little  sensibility, 
in  order  that  it  might  be  employed  for  determining  the  relative  power 
while  the  observer  was  but  a  few  yards  from  the  machine. 

These  experiments  have  been  made  in  an  apartment  of  less  than  80 
feet  in  length,  in  which  the  sounding  apparatus  was  placed  at  one  end 


476  EESEARCHES   IN   SOUND. 

and  the  artificial  ear  at  the  other,  substituting  fine  shot  instead  of  sand. 

The  experiments  with  the  siren  however  indicate  the  fact  that  neither 
the  highest  nor  the  lowest  pitch  of  an  instrument  gives  the  greatest  pen- 
etrating power,  but  one  of  a  medium  character. 

Another  element  of  importance  in  the  construction  of  these  instru- 
ments is  the  volume  of  sound.  To  illustrate  this,  it  may  be  mentioned 
that  a  harpsichord- wire  stretched  between  two  strings  of  India  rubber, 
when  made  to  vibrate  by  means  of  a  fiddle-bow,  gives  scarcely  any 
appreciable  sound.  We  attribute  this  to  the  want  of  quantity  in  the 
aerial  wave;  for  if  the  same  wire  be  stretched  over  a  sounding-board 
having  a  wide  area,  the  effect  will  be  a  comparatively  loud  sound,  but 
of  less  duration,  with  a  given  impulse.  It  was  therefore  suggested  that 
the  width  of  the  reed  in  the  Daboll  trumpet,  the  form  and  size  of  the 
holes  in  the  disk  of  the  siren,  and  the  circumference  of  the  vibrating 
sheet  of  air  issuing  from  the  circular  orifice  of  the  whistle,  would  affect 
the  power  of  the  sound.  The  only  means  of  testing  this  suggestion  is 
by  using  reeds  of  different  widths,  sirens  with  disks  of  different-shaped 
openings,  and  whistles  of  different  diameters.  In  conformity  with  this 
view,  Mr.  Brown  has  made  a  series  of  empirical  experiments  with  open- 
ings of  different  forms,  which  have  greatly  improved  the  operation  of 
the  siren,  while  Mr.  Wilcox  has  experimented  on  several  forms  of  reeds, 
of  which  the  following  is  the  result : 

The  best  reed  obtained  was  2J  inches  wide,  8  inches  long  in  the  vi- 
brating part,  |  inch  thick  at  the  butt,  and  J  inch  thick  at  the  free  end. 
This  sounded  at  a  pressure  of  from  20  to  30  pounds.  The  thinner  reeds 
gave  a  sound  at  a  less  pressure,  from  5  to  f  0  pounds,  the  thicker  at  from 
20  to  30  pounds.  A  reed  8.J-  inches  long  in  the  vibrating  part,  1  inch  thick 
at  the  butt,  f  inch  thick  at  the  end,  and  3  inches  wide,  did  not  begin  to 
sound  until  a  pressure  of  80  pounds  was  reached,  then  gave  a  sound  of 
a  dull  character.  Another  reed  of  the  same  width,  -§  inch  thick  at  the 
butt,  and  -fa  inch  at  the  end,  and  same  length,  gave  a  sound  at  75  pounds 
pressure,  but  still  dull  and  of  little  penetrating  power.  These  reeds 
were  evidently  too  heavy  in  proportion  to  their  elasticity.  These  were 
made  without  the  addition  of  a  trumpet,  and  therefore  to  produce  the 
best  result  when  used  with  a  trumpet,  the  latter  must  be  increased  or 
diminished  in  length  until  its  natural  vibrations  are  in  harmou3r  with 
those  of  the  former,  as  will  be  seen  hereafter.  General  Duane  has  also 
made  experiments  on  whistles  of  different  diameters,  of  which  the  result 
will  be  given. 

Another  consideration  in  regard  to  the  same  matter  is  that  of  the  am- 
plitude of  the  oscillations  of  the  tongue  or  steel  reed  in  its  excursion  in 
producing  the  sound ;  the  time  of  oscillation  remaining  the  same,  that 
is,  the  pitch,  the  amplitude  will  depend  upon  the  elasticity  of  the  reed, 
the  power  to  surmount  which  will  again  depend  upon  the  pressure  of 
steam  in  the  boiler,  and  hence  we  might  infer  that  an  increase  of  pres- 


RESEARCHES    IN    SOUND.  477 

sure  in  the  boiler  with  an  increase  of  the  elasticity  of  the  reed,  every- 
thing else  being  the  same,  would  produce  an  increase  in  penetrating 
power.  From  the  general  analogy  of  mechanical  effects  produced  by 
motive  power,  we  may  denote  the  effect  upon  the  ear  by  the  expression 
mv*,  in  which  m  expresses  the  mass  or  quantity  of  air  in  motion,  and  v 
the  velocity  of  the  particles  in  vibration. 

If  this  be  the  expression  for  the  effect  upon  the  ear,  it  is  evident  that 
in  case  of  a  very  high  note  the  amplitude  of  the  vibration  must  be  so 
small  that  the  effect  would  approximate  that  of  a  continued  pressure 
rather  than  that  of  distinct  alternations  of  pressure,  giving  a  vibrating 
motion  to  the  drum  of  the  ear. 

4.  Next,  experiments  were  made  to  determine  the  penetrating  power  in 
the  case  of  the  siren  under  different  pressures  of  steam  in  the  boiler. 
The  experiments  commenced  with  a  pressure  of  100  pounds.  The  pres- 
sure at  each  blast  was  noted  by  two  observers,  and  to  compare  these 
pressures  with  the  indications  of  the  sand,  the  time  of  the  blasts  was 
also  noted. 

The  following  are  the  results : 

Pressure.  Relative  distances  nL,  which 

the  sand  ceased  to  move. 

100 61 

90 59 

80 58 

70 57 

60 57 

50 56 

40 55 

30 53 

20 51 

From  this  series  of  experiments,  it  appears  that  a  diminution  of  pres- 
sure is  attended  with  a  comparatively  small  diminution  in  the  pene- 
trating power  of  the  siren. 

In  regard  to  this  unexpected  result  of  great  practical  importance,  the 
following  appears  to  be  the  explanation.  It  is  a  well-known  principle 
in  aerial  mechanics  that  the  velocity  of  the  efflux  of  air  from  an  orifice 
in  a  reservoir  does  not  increase  with  an  increase  of  condensation,  when 
the  spouting  is  into  a  vacuum.  This  is  evident  when  we  reflect  that 
the  weight  of  density  of  the  air  moving  out  is  increased  in  proportion 
to  the  elasticity  or  pressure;  that  is,  the  increase  in  the  propelling  force 
is  proportional  to  the  increase  in  the  weight  to  be  moved,  hence  the 
velocity  must  remain  the  same. 

In  the  foregoing  experiments  with  high  pressures  large  in  proportion 
to  the  resistance  of  the  air,  the  velocity  of  efflux  should  therefore  be 
but  little  increased  with  the  increase  of  pressure,  and  inasmuch  as  the 
velocity  is  the  roost  important  factor  in  the  expression  mv'2,  which  indi- 
cates the  effect  on  the  tympanum,  the  penetrating  power  of  the  sound 
should  be  in  accordance  with  the  above  experimental  results. 


478  RESEARCHES   IN   SOUND. 

A  similar  result  cannot  be  expected  with  the  use  of  the  whistle  or 
the  trumptt,  since  in  the  former  the  stiffness  of  the  aerial  reed  depends 
upon  its  density,  which  will  be  in  proportion  to  the  pressure  in  the 
boiler,  and  in  the  case  of  the  latter  no  sound  can  be  produced  on  the 
one  hand  unless  the  pressure  be  sufficient  to  overcome  the  resistance  of 
the  reed,  and  on  the  other  the  sound  must  cease  when  the  pressure  is 
so  great  as  to  prevent  the  recoil  of  the  reed. 

5.  An  experiment  was  made  to  determine  the  effect  of  a  small  whistle 
inserted  kico  the  side  of  a  trumpet  near  the  small  end.    The  whistle 
being  sounded  before  and  after  it  was  placed  in  the  trumpet,  the  result 
was  as  follows:  The  penetrating  powers  were  in  the  ratio  of  40:  51, 
while  the  tone  was  considerably  modified.     From  this  experiment  it 
appears  that  a  whistle  may  be  used  to  actuate  a  trumpet  or  to  exercise 
the  functions  of  a  reed.    In  order  however  to  get  the  best  results,  it 
would  be  necessary  that  the  trumpet  and  whistle  should  be  in  unison, 
but  it  may  be  doubted  however  whether  an  increase  of  effect,  with  a 
given  amount  of  power,  would  result  from  using  such  an  arrangement; 
it  might  nevertheless  be  of  advantage  in  certain  cases  to  direct  the 
sound  of  a  locomotive   in  a  definite  direction,  and  to  use   a  smaller 
whistle,  especially  in  cities,  in  which  the  locomotive  passes  through  long- 
streets;  perhaps  in  this  case  the  sound  might  be  less  disagreeable  than 
that  of  the  naked  whistle,  which  sends  its  sound-waves  laterally  with 
as  much  force  as  in  the  direction  of  the  motion  of  the  engine. 

6.  General  Poe  called  attention  to  the  sound  produced  by  the  paddle- 
wheels  of  a  steamer  in  the  ofh'ng  at  a  distance  estimated  at  four  and  a 
half  miles.    The  sound  was  quite  distinct  when  the  ears  were  brought 
near  the  surface  of  the  beach. 

In  this  connection  he  stated  that  he  had  heard  the  approach  of  a 
small  steamer  on  the  northern  lakes  when  its  hull  was  still  below  the 
horizon,  and  was  even'enabled  to  designate  the  particular  vessel  from 
among  others  by  the  peculiarity  of  the  sound. 

The  sound  in  the  case  of  the  steamer  is  made  at  the  surface  of  the 
water,  and  it  might  be  worth  the  trouble  to  try  experiments  as  to  the 
transmission  of  sound  under  this  condition,  and  the  collection  of  it  by 
means  of  ear-trumpets,  the  mouths  of  which  are  near  the  water,  the 
sound  being  conveyed  through  tubes  to  the  ears  of  the  pilot.  In  order 
however  to  determine  in  this  case  the  direction  of  the  source  of  sound, 
two  trumpets  would  be  necessary,  one  connected  with  each  ear,  since 
we  judge  of  the  direction  of  a  sound  by  its  simultaneous  effects  on  the 
two  auditory  nerves.  This  suggestion,  as  well  as  many  others  which 
have  occurred  in  the  course  of  these  researches,  is  worthy  of  special  in- 
vestigation. 

7.  A  series  of  experiments  was  made  to  compare  trumpets  of  different 
materials  and  forms  having  the  same  length  and  transverse  areas,  all 
blown  at  a  pressure  of  9 J  pounds. 


EESEARCHES    IN   SOUND. 
The  following  table  gives  the  results  : 


479 


No. 

Material  of  trumpet. 

Cross-section. 

Relative  distances  at  which 
the  sand  ceased  to  move. 

1 

2 
3 
4 

Wood. 
Brass. 
Cast  iron. 
Wood. 

Square. 
Circular. 
Circular. 
Circular. 

13 
23 
24 
30 

From  these  experiments  it  would  appear  that  the  material  or  elasti- 
city of  the  trumpet  had  little  or  no  effect  on  the  penetrating  power  of 
the  sound,  although  the  shape  appeared  to  have  some  effect,  the  pyri- 
midal  trumpet,  or  one  with  square  cross-section  (No.  1),  giving  a  less  re- 
sult than  the  conical  ones  of  the  same  sectional  area.  A  comparison 
was  made  between  a  long  straight  trumpet  and  one  of  the  same  length 
curved  at  its  upper  end,  which  gave  the  same  penetrating  power  with 
the  same  pressure.  It  is  probable  that  a  thin  metallic  trumpet  would 
give  greater  lateral  divergency  to  the  sound,  and  also  a  slightly  differ- 
ent tone. 

8.  The  effect  of  a  hopper-formed  reflector  was  next  tried  with  the 
whistle,  the  axis  of  which  was  about  5  feet  in  length,  the  mouth  6  feet 
square,  and  the  small  end  about  18  inches.    When  the  whistle  was 
sounded  at  the  small  end  of  this  reflector,  the  distance  at  which  the 
sand  ceased  to  move  was  51 ;  the  sound  of  the  same  whistle  without  the 
reflector  ceased  to  move  the  sand  at  40.    The  ratio  of  these  distances 
would  have  been  less  with  a  more  sensitive  instrument  at  a  greater  dis- 
tance on  account^  of  the  divergency  of  the  rays. 

9.  In  order  to  determine  the  diminution  of  sound  by  departing  from 
the  axis  of  the  trumpet,  a  series  of  experiments  was  made  with  a  ro- 
tating trumpet,  the  axis  of  which  was  at  first  directed  along  the  gradu- 
ated line  of  observation,  and  subsequently  deflected  from  that  line  a 
given  number  of  degrees.    The  following  were  the  results : 


Direction  of  the  trumpet. 


Relative  distance  at 
which  sand  moved. 


Along  the  line  

26 

Deflected  30°  

23 

Deflected  60°  fc  

21 

Deflected90o  

18 

Deflected  120°  

13 

These  results  illustrate  very  strikingly  the  tendency  of  sound  to  spread 
on  either  side  of  the  axis  of  the  trumpet ;  had  the  experiments  been  made 
with  a  more  sensitive  instrument,  and  at  a  greater  distance,  the  effect 


480  RESEARCHES   IN   SOUND. 

would  have  shown  a  much  greater  divergency.  It  should  be  observed 
however  that  the  mouth  of  the  trumpet  in  this  case  was  36  inches,  which 
is  unusually  large. 

From  the  experiments  made  near  New  Haven,  and  also  from  those  at 
this  station,  it  appears  that  the  actual  amount  of  power  to  produce  sound 
of  a  given  penetration  is  absolutely  less  with  a  reed  trumpet  than  with  a 
locomotive  whistle.  This  fact  probably  finds  its  explanation  in  the  cir- 
cumstance that  in  each  of  these  instruments  the  loudness  of  the  sound 
is  due  to  the  vibration  of  the  air  in  the  interior  of  the  trumpet  and  in 
the  bell  of  the  whistle,  each  of  these  being  a  resounding  cavity ;  and 
furthermore  that  in  these  cavities  the  air  is  put  in  a  state  of  sustained 
vibration  by  the  undulations  of  a  tongue,  in  the  one  case  of  metal,  in 
the  other  of  air  ;  and  furthermore  it  requires  much  more  steam  to  set 
the  air  in  motion  by  the  tongue  of  air  than  by  the  solid  tongue  of  steel, 
the  former  requiring  a  considerable  portion  of  the  motive  power  to  give 
the  current  of  which  it  consists  the  proper  degree  of  stiffness,  if  I  may 
use  the  word,  to  produce  the  necessary  rapidity  of  oscillation.  But  what- 
ever may  be  said  in  regard  to  this  supposition,  it  is  evident,  in  case  reli- 
able hot-air  engines  cannot  be  obtained,  that  the  Daboll  trumpet  may  be 
operated  by  a  steam-engine,  although  at  an  increased  cost  of  mainte- 
nance, but  this  increase  we  think  will  still  not  be  in  proportion  to  the 
sound  obtained  in  comparison  with  the  whistle. 

Another  question  which  naturally  arises,  but  which  has  not  yet  been 
definitely  settled  by  experment,  is  whether  both  the  siren  and  the  whis- 
tle would  not,  equally  with  the  trumpet,  give  more  efficient  results  when 
worked  by  condensed  air  than  by  steam. 

From  hypothetical  considerations  this  would  appear  to  be  the  case, 
since  the  intensity  of  sound  depends  upon  the  density  of  the  medium  in 
which  it  is  produced;  and  as  the  steam  is  considerably  lighter  than  air, 
and  as  the  cavities  of  all  of  these  instruments  are  largely  filled  with  steam, 
the  intensity  of  sound  would,  on  this  account,  seem  to  be  less  than  if 
filled  with  air. 

At  the  conclusion  of  the  experiments  at  Sandy  Hook,  the  siren  was 
adopted  as  a  fog-signal,  in  addition  to  the  reed-trumpet  and  the  locomo- 
tive-whistle, to  be  applied  to  the  more  important  stations,  while  large 
bells  were  retained  for  points  at  which  fog-signals  were  required  to  be 
heard  at  but  comparatively  small  distances.  These  instruments  of  the 
first  class  being  adopted,  it  became  of  importance  to  determine,  in  actual 
practice,  the  cost  of  maintenance,  the  best  method  of  working  them,  and 
any  other  facts  which  might  have  a  bearing  on  their  use. 

But  as  investigations  of  this  kind  would  require  much  time  and  pecu- 
liar advantages  as  to  location  and  mechanical  appliances,  this  matter 
was  referred  to  General  Duane,  the  engineer  in  charge  of  the  1st  and 
2d  light-house  districts,  who  had  peculiar  facilities  near  his  residence, 
at  Portland,  Me.,  in  the  way  of  workshops  and  other  conveniences,  and 
who,  from  his  established  reputation  for  ingenuity  and  practical  skill 


RESEARCHES    IX    SOUND.  481 

ill  mechanism,  was  well  qualified  for  the  work.  The  assignment  of  this 
duty  to  General  Duane  by  the  Light-House  Board  was  made  during  my 
absence  in  Europe,  in  1870,  and  as  my  vacation  in  1871  was  devoted  to 
light-house  duty  in  California,  I  had  no  opportunity  of  conferring  with 
him  on  the  subject  until  after  his  experiments  were  completed.  His  re- 
sults are  therefore  entirely  independent  of  those  obtained  under  my  di- 
rection, and  I  give  them  herewith  in  his  own  words,  with  such  com- 
ments as  they  may  suggest  and  as  are  necessary  to  a  proper  elucidation 
of  the  subject. 


EXPERIMENTS  AT  PORTLAND,  ME.    1871,  BY  GENERAL  DUANE. 

The  apparatus  employed  consisted  of  the  first-class  siren,  first-class  Daboll  trumpet 
and  steam-whistles  of  various  sizes. 

The  points  to  he  decided  were : 

1st.  The  relative  power  of  these  machines ;  i.  e.,  the  distances  at  which  they  could  be 
heard  under  various  conditions  of  the  atmosphere. 

2d.  The  amount  of  fuel  and  water  consumed  by  each. 

3d.  The  attention  and  skill  required  in  operating  them. 

4th.  Their  endurance. 

5th.  Whether  they  are  sufficiently  simple  in  construction  to  permit  of  their  being 
managed  and  kept  in  running  order  by  the  class  of  men  usually  appointed  light- 
house keepers. 

In  conducting  these  experiments  the  following  method  was  pursued  : 

The  signals  were  sounded  at  alternate  minutes,  and  their  sound  compared  at  dis- 
tances of  two,  three,  and  four  miles,  and  from  different  directions.  On  every  occasion 
the  quantity  of  fuel  and  water  consumed  per  hour  by  each  was  carefully  noted,  and 
the  condition  of  each  machine  examined,  both  before  and  after  the  trial,  to  ascertain 
whether  any  of  its  parts  had  sustained  injury. 

Before  giving  the  results  of  these  experiments  some  facts  should  be  stated,  which 
will  explain  the  difficulty  of  determining  the  power  of  a  fog-signal. 

There  are  six  steam  fog- whistles  on  the  coast  of  Maine  ;  these  have  been  frequently 
heard  at  a  distance  of  twenty  miles,  and  as  frequently  cannot  be  heard  at  the  distance 
of  two  miles,  and  this  with  no  perceptible  difference  in  the  state  of  the  atmosphere. 

The  signal  is  often  heard  at  a  great  distance  in  one  direction,  while  in  another  it 
will  be  scarcely  audible  at  the  distance  of  a  mile.  This  is  not  the  effect  of  wind,  as 
the  signal  is  frequently  heard  much  farther  against  the  wind  than  with  it.  For  ex- 
ample, the  whistle  on  Cape  Elizabeth  can  always  be  distinctly  heard  in  Portland,  a 
distance  of  nine  miles,  during  a  heavy  northeast  snow-storm,  the  wind  blowing  a  gale 
dieectly  from  Portland  toward  the  whistle, 

[In  this  sentence,  General  Duane  certainly  does  not  intend  to  convey 
the  idea  that  a  signal  is  frequently  heard  "  at  a  much  greater  distance 
against  the  wind  than  with  it,"  since  this  assertion  would  be  at  variance 
with  the  general  experience  of  mankind;  but  the  word  " frequently " 
applies  to  the  whistle  on  Cape  Elizabeth,  which  has  been  already  men- 
tioned as  a  remarkably  exceptional  case,  in  which  the  sound  is  heard 
best  against  the  wind  during  a  northeast  snow-storm.] 

The  most  perplexing  difficulty,  however,  arises  from  the  fact  that  the  signal  often 
appears  to  be  surrounded  by  a  belt,  varying  in  radius  from  one  to  one  and  a  half  miles, 
from  which  the  sound  appears  to  be  entirely  absent.  Thus,  in  moving  directly  from 
a  station,  the  sound  is  audible  for  the  distance  of  a  mile,  is  then  lost  for  about  the 
same  distance,  after  which  it  is  again  distinctly  heard  for  a  long  time.  This  action  is 
common  to  all  ear-signals,  and  has  been  at  times  observed  at  all  the  stations,  at  one 
of  which  the  signal  is  situated  on  a  bare  rock  twenty  miles  from  the  main -land,  with 
no  surrounding  objects  to  affect  the  sound. 

All  attempts  to  re-enforce  the  sound  by  means  of  reflectors  have  hitherto  been  un- 
successful. Upon  a  large  scale,  sound  does  not  appear,  on  striking  a  surface,  to  be 
reflected  after  the  manner  of  light  and  heat,  but  to  roll  along  it  like  a  clpud  of  smoke. 
S.  Ms.  52 31 


482  RESEARCHES    IN    SOUND. 

[This  statement  is  in  a  measure  in  accordance  with  results  which  I 
have  previously  found  in  connection  with  investigations  at  the  light- 
house near  New  Haven,  in  which  the  conclusion  was  arrived  at,  that 
although  rays  of  feeble  sounds,  and  for  a  short  distance,  observe  the 
law  that  the  angle  of  reflection  is  equal  to  the  angle  of  incidence  after 
the  manner  of  light,  yet  powerful  sounds  tend  to  diverge  laterally  to 
such  a  degree  as  to  render  reflectors  of  comparatively  little  use.] 

In  view  of  these  circumstances,  it  will  be  obvious  that  it  was  extremely  difficult  to 
determine  the  extent  of  the  power  of  the  various  signals  under  examination. 

It  should  be  remembered  that  while  the  sound  from  the  whistle  is  equally  distrib- 
uted in  all  directions,*  that  from  the  two  other  signals,  both  of  which  tire  provided 
with  trumpets,  is  not  so  distributed. 

[The  difference  is  apparent  near  by,  but,  as  we  have  seen  before,  on 
account  of  the  tendency  of  sound  to  spread  it  is  imperceptible  at  a  dis- 
tance.] 

In  the  siren  the  sound  is  most  distinct  in  tbjs  axis  of  the  trumpet. 

In  the  Daboll  trumpet  it  is  usually  strongest  in  a  plane  perpendicular  to  this  axis. 

[This  is  at  variance  directly  with  any  observation  I  have  myself  made.] 

Relative  power. — From  the  average  of  a  great  number  of  experiments  the  following 
result  was  obtained : 

The  power  of  the  first-class  siren,  12"  whistle,  and  first-class  Daboll  trumpet,  may 
be  expressed  by  the  numbers  9,  7,  4. 

The  extreme  limit  of  sound  of  the  siren  was  not  ascertained.  That  of  the  12"  whistle 
is  about  twenty  miles,  and  of  the  trumpet  twelve. 

Consumption  of  fuel  and  water. — The  siren,  when  working  with  a  pressure  of  72  pounds 
of  steam,  consumes  about  180  pounds  of  coal  and  126  gallons  of  water  per  hour. 

The  12"  whistle,  with  55  pounds  pressure  of  steam,  consumes  CO  pounds  of  coal  and 
40  gallons  of  water  per  hour. 

The  Daboll  trumpet,  with  10  pounds  pressure  of  air  in  the  tank,  consumes  about  20 
pounds  of  coal  per  hour. 

The  relative  expenditure  of  fuel  would  be :  siren,  9 ;  whistle,  3 ;  trumpet,  1. 

The  siren. — Of  the  three  machines  this  is  the  most  complicated.  It  uses  eteam  at  a 
high  pressure,  and  some  of  its  parts  move  with  very  great  velocity,  the  siren  spindle 
making  from  1,800  to  2,400  revolutions  per  minute.  The  boiler  must  be  driven  to  its 
full  capacity  in  order  to  furnish  sufficient  steam.  A  large  quantity  of  strain  is,  at 
intervals,  suddenly  drawn  from  the  boiler,  causing  a  tendency  to  foain,  and  to  eject  a 
considerable  amount  of  water  through  the  trumpet. 

The  constant  attention  of  the  keeper  is  required  to  regulate  the  fire,  the  supply  of 
water  to  the  boiler,  of  oil  to  the  journals,  &c. 

In  general  terms,  it  may  be  stated  that  the  siren  requires  more  skill  and  attention 
in  its  management  than  either  of  the  other  signals. 

The  Daboll  trumpet. — As  the  caloric  engine,  which  has  been  hitherto  employed  to 
operate  this  signal,  requires  little  fuel,  no  water,  and  is  perfectly  safe  as  regards 
danger  from  explosion,  it  would,  at  the  first  glance,  appear  to  be  the  most  suitable 
power  that  could  be  applied  to  fog-signals,  and  was  accordingly  at  first  exclusively 
adopted  for  this  purpose.  It  was  however  found  to  be  so  liable  to  accident  and  so 
difficult  to  repair  that  of  late  years  it  has  been  almost  entirely  rejected.  In  the  steam- 
boiler  the  furnace  is  surrounded  by  water,  and  it  is  impossible,  under  ordinary  circum- 
stances, to  heat  the  metal  much  above  the  temperature  of  the  water.  The  furnace  of 
the  caloric  engine  is  surrounded  by  air,  and  is  therefore  liable  to  be  burned  out  if  the 
fire  is  not  properly  regulated. 

The  working-piston  is  packed  with  leather,  and  as  it  moves  horizontally,  with  ita 
whole  weight  resting  on  the  lower  side  of  the  cylinder,  the  packing  at  its  lower  edge 
is  soon  worn  out. 

If  the  engine  is  allowed  to  stop  with  the  piston  at  the  furnace-end  of  the  cylinder, 

*The  sound  of  the  whistle  is  equally  distributed  horizontally.  It  is,  however, 
much  stronger  in  the  plane  containing  the  lower  edge  of  the  bell  than  on  either  side 
of  this  plane.  Thus,  if  the  whistle  is  standing  upright,  in  the  ordinary  position,  its 
sound  is  more  distinct  in  a  horizontal  plane  passing  through  the  whistle  than  above 
or  below  it. 


EESE ARCHES    IN    SOUND.  483 

the  leather  is  destroyed  by  the  heat.  The  repacking  of  a  piston  is  a  difficult  and 
expensive  operation,  requiring  more  skill  than  can  be  expected  among  the  class  of 
men  from  whom  light-house  keepers  are  appointed. 

Another  accident  to  which  these  engines  are  subject  arises  from  a  sudden  check  in 
the  velocity  of  the  piston,  caused  either  by  the  jamming  of  the  leather  packing  or  the 
introduction  of  dirt  into  the  open  end  of  the  cylinder,  in  which  case  the  momentum  of 
the  heavy,  eccentrically-loaded  fly-wheel  is  almost  sure  to  break  the  main  rocker-shait. 

The  expense  of  repairs  is  considerably  increased  by  the  fact  that  these  engines  are 
not  now  in  general  use,  and  when  important  repairs  are  required  it  is  usually  neces- 
sary to  send  to  the  manufacturer. 

This  signal  requires  much  attention.  The  fires  must  be  carefully  regulated  to  avoid 
burning  out  the  furnace,  the  journals  thoroughly  oiled,  and  the  cylinders  well  sup- 
plied with  tallow. 

The  steam-whistle. — This  machine  requiring  much  less  steam  than  the  siren  in  propor- 
tion to  the  size  of  its  boiler,  there  is  not  the  same  necessity  for  forcing  the  fire ;  the 
pressure  of  steam  required  is  less,  and  the  point  from  which  it  is  drawn  much  higher 
above  the  water-level  in  the  boiler,  and  there  is  consequently  no  tendency  to  foam. 

The  machinery  Ls  simple ;  the  piston  pressure  very  light,  producing  but  little  strain 
on  the  different  parts  of  the  engine,  which  is  therefore  not  liable  to  get  out  of  order 
and  requires  no  more  attention  than  a  common  stationary  engine. 

One  marked  advantage  possessed  by  this  signal  is  that  should  the  engine  become 
disabled,  the  whistle  may  still  be  sounded  by  working  the  valve  by  hand.  This  is  not 
the  case  with  the  two  others,  where  an  accident  to  any  part  of  the  machinery  renders 
the  signal  for  the  time  useless. 

It  will  thus  be  seen  that  the  siren  is  the  most  expensive  of  the  fog-signals  as  regards 
maintenance,  and  that  it  is  adapted  only  to  such  stations  as  are  abundantly  supplied 
with  water  and  situated  in  the  vicinity  of  machine-shops  where  the  necessary  repairs 
can  be  promptly  made. 

On  the  other  hand,  as  it  is  the  most  powerful  signal,  there  are  certain  stations  where 
it  should  have  the  preference ;  as,  for  example,  Sandy  Hook,  which  from  its  import- 
ance demands  the  best  signal  that  can  be  procured,  regardless  of  cost.  Such  stations 
should  be  provided  with  duplicate  apparatus,  well  supplied  with  spare  parts,  to  guard 
against  any  possibility  of  accident. 

There  should  be  a  keeper  whose  sole  business  must  be  to  attend  the  signal,  and  who 
should  have  sufficient  mechanical  skill  to  make  the  ordinary  repairs.  He  should 
moreover  be  a  licensed  engineer. 

There  will  also  be  required  an  assistant,  who  may  be  one  of  the  light-keepers,  to 
relieve  him  during  the  continuance  of  foggy  weather. 

The  steam- whistle  is  the  simplest  in  construction,  most  easily  managed  and  kept  in 
repair,  and  requires  the  least  attention  of  all  the  fog-signals.  It  is  sufficiently  power- 
ful for  most  localities,  while  its  consumption  of  fuel  and  water  is  moderate. 

It  has  been  found  on  this  coast  that  a  sufficient  quantity  of  rain-water  can  be  col- 
lected to  supply  the  12"  whistle  at  nearly  every  station.  This  has  been  the  case  for 
the  last  two  years  at  Martinicus. 

The  Daboll  trumpet,  operated  by  a  caloric  engine,  should  only  be  employed  in  ex- 
ceptional cases,  such  as  at  stations  where  no  water  can  be  procured,  and  where,  from, 
the  proximity  of  other  signals,  it  may  be  necessary  to  vary  the  nature  of  the  sound. 

The  trumpet  however  may  undoubtedly  be  very  much  improved  by  employing 
steam  power  for  condensing  the  air.  The  amount  of  work  required,  which  is  that  of 
compressing  70  cubic  feet  of  air  to  an  average  pressure  of  8  pounds  per  inch,  would  be 
less  than  two-horse  power.  For  this  purpose  the  expenditure  of  fuel  and  water  would 
be  moderate ;  indeed,  the  exhaust  steam  could  be  condensed  and  returned  to  the  cis- 
tern, should  the  supply  of  water  be  limited. 

The  siren  also  is  susceptible  of  improvement,  especially  as  regards  simplification. 

[In  the  foregoing  remarks  we  think  the  general  has  expressed  a  some- 
what undue  partiality  for  the  whistle,  and  somewhat  overestimated  the 
defects  of  the  other  instruments.  The  trumpets,  with  Ericsson  engine, 
have  not  been  abandoned,  except  partially  in  the  two  districts  under 
the  direction  of  General  Duane,  to  which  he  probably  intended  to  con- 
fine his  statement.  They  are  still  in  use  in  the  third  district,  where 
they  are  preferred  by  General  Woodruff,  who  finds  no  diflicultyin  keep- 
ing them  in  repair,  having  employed  a  skilled  machinist  who  has  made 
these  instruments  his  special  study,  and  who,  visiting  them  from  time 
to  time,  makes  repairs  and  supplies  new  parts.] 


484  KESEARCHES    IN    SOUND. 

Th^  intermittent  action  of  fog-signals  makes  it  necessary  to  employ  a  peculiar  form 
of  boiler.  The  steam  used  is  at  a  high  pressure,  and  drawn  off  at  intervals;  conse- 
quently there  is  a  tendency  to  foam  .and  throw  out  water  with  the  steam.  To  obviate 
this  difficulty  the  form  of  boiler  found  by  experience  to  be  best  adapted  to  this  service 
is  a  horizontal  tubular  boiler  (locomotive),  with  rather  more  than  one-half  of  the  inte- 
rior space  allowed  for  steam-room.  The  steam-dome  is  very  large,  and  is  surmounted 
by  a  steam  pipe  12"  in  diameter.  Both  the  dome  and  pipe  were  formerly  made  much 
smaller,  but  were  gradually  enlarged  as  long  as  any  difficulty  with  regard  to  foaming 
was  noticed.  The  steam  is  drawn  off  at  a  point  10"  above  the  water-level  in  the 
boiler.  The  main  points  to  be  observed  are  to  have  plenty  of  steam-room,  and  to  draw 
the  steam  from  a  point  high  above  the  water-level.  It  will  be  readily  perceived  that 
a  vertical  tubular  boiler  is  entirely  unsuited  to  this  work. 

It  is  essential,  both  as  regards  economy  of  fuel  and  the  efficient  working  of  the  sig- 
nal, that  the  boiler,  including  the  dome  and  stand-pipe,  should  be  well  covered  with 
some  good  non-conductor  of  heat.  A  material,  called  salamander  felting,  manufac- 
tured in  Troy,  N.  Y.,  was  used  on  the  fog- whistle  boiler  at  House  Island  during  the 
winter  of  1870.  There  resulted  a  saving  of  more  than  20  per  cent,  of  fuel  over  that 
consumed  in  the  same  boiler  when  uncovered.  Where  this  material  cannot  be  pro- 
cured, a  thick  layer  of  hair  felting,  covered  with  canvas,  will  be  found  to  answer  a 
good  purpose. 

Various  expedients  have  been  proposed  with  the  view  of  keeping  the  water  in  the 
boilers  hot  when  the  signals  are  not  in  operation,  that  the  signal  may  always  be  ready 
to  sound  at  a  very  short  notice,  and  that  the  water  in  the  boiler  and  pipes  may  be 
prevented  from  freezing  in  extremely  cold  weather.  One  of  these  contrivances  is 
"  Button's  circulating  water-heater."  It  consists  essentially  of  a  small,  vertical,  tubu- 
lar boiler,  entirely  filled  with  water,  and  connected  with  the  boiler  or  tank  which 
contains  the  water  to  be  heated,  by  two  pipes  on  different  levels.  As  soon  as  the 
water  in  the  heater  is  warmed,  a  circulation  commences,  the  hot  water  flowing  through 
the  upper  pipe  into  the  boiler,  and  the  cold  through  the  lower  pipe  from  the  boiler  to 
the  heater.  As  the  furnace  in  the  heater  is  very  small  but  little  fuel  is  consumed,  and 
nearly  the  entire  heat  produced  by  the  combustion  is  utilized. 

The  apparatus  has  been  extensively  employed  in  heating  the  water  in  tanks  designed 
frr  filling  the  steam  fire-engine  boilers,  when  the  alarm  of  fire  is  first  given,  and  appears 
admirably  adapted  to  this  purpose.  If  used  in  connection  with  a  steam-boiler,  it  should 
be  disconnected  before  steam  is  raised  in  the  latter,  as,  from  its  construction,  it  is  not 
calculated  to  withstand  any  considerable  pressure. 

An  arrangement,  similar  in  principle,  has  been  used  in  the  first  light-house  district, 
consisting  of  a  small  cylinder  coal-stove,  of  the  ordinary  pattern,  around  the  interior 
of  which,  and  above  the  grate,  is  introduced  a  single  coil  of  f"  pipe.  This  coil  is 
connected  with  the  boiler  by  two  pipes,  one  entering  near  the  bottom,  the  other  about 
2  feet  higher.  It  has  been  found  that  in  consequence  of  the  rapid  circulation  of  the 
water  through  this  coil,  and  the  great  capacity  of  water  for  heat,  that  nearly  all  the 
heat  from  the  fire  in  the  stove  is  transferred  to  the  water  in  the  boiler.  This  arrange- 
ment possesses  the  advantage  of  the  £ "  pipe,  being  strong  enough  to  stand  any  press- 
ure that  can  be  used  in  the  boiler,  rendering  it  unnecessary  to  disconnect  it  at  any  time. 

Experience  has,  however,  proved  that  none  of  these  contrivances  are  essential.  It 
is  seldom  that  an  attentive  keeper  cannot  foresee  the  approach  of  fog  or  snow  in  time 
to  have  the  apparatus  in  operation  as  soon  as  required,  even  when  obliged  to  start  his 
fire  with  cold  water  in  the  boiler. 

Keepers  should  be  directed  to  watch  the  state  of  the  weather  carefully,  and  to  light 
their  tires  at  the  first  indication  of  fog  or  snow-storm.  As  soon  as  the  water  in  the 
boiler  is  near  the  boiling  point,  should  the  necessity  for  sounding  the  signal  have  not 
yet  arisen,  the  fire  may  be  banked,  and  in  this  state  the  water  may  be  kept  hot  for  any 
length  of  time  at  a  moderate  expenditure  of  fuel.  With  proper  care,  no  more  fuel  is 
required  to  keep  the  water  at  the  requisite  temperature  by  means  of  a  banked  fire  than 
by  any  other  method,  and  it  is  a  matter  of  great  importance  to  avoid  complicating 
fog-signal  apparatus  by  unnecessary  appendages. 

The  same  plan  should  be  adopted  in  extremely  cold  weather  to  prevent  the  water 
iu  the  boiler  from  freezing.  There  should  be  a  small  air-cock  in  the  draught-pipe  near 
its  junction  with  the  feed-pump,  and  in  cold  weather  this  should  be  opened  when  the 
pump  is  not  in  use,  in  order  to  allow  the  pipe  to  empty  itself. 

When  the  draught-pipe  cannot  be  protected  from  the  cold,  and  the  well  is  at  a  con- 
siderable distance  from  the  engine,  the  following  expedient  has  been  employed  with 
success :  The  pipe  is  inclosed  in  an  India-rubber  hose  of  about  double  its  diameter, 
and  from  time  to  time  steam  is  forced  through  the  space  between  the  hose  and  draught- 
pipe  by  means  of  a  small  pipe  from  the  boiler. 

Although  the  laws  governing  the  reflection  of  light  and  heat  are  undoubtedly,  in  ua 
great  measure,  applicable  to  sound,  there  are  yet  so  many  disturbing  influences,  such  as 
inflection,  refraction,  caused  by  the  varying  density  of  the  atmosphere,  &c.,  interfering 
with  the  reflection  of  the  latter,  that  but  little  use  can  be  made  of  this  property  in 


RESEARCHES   IN   SOUND.  485 

directing  and  condensing  the  waves  of  sound  issuing  from  a  fog-signal.  This  fact  may 
be  illustrated  by  an  account  of  some  experiments  made  during  the  last  year. 

A  whistle  being  sounded  in  the  focus  of  a  large  parabolic  reflector,  it  was  very  per- 
ceptible to  an  observer  in  the  immediate  vicinity  that  the  sound  was  louder  in  the 
front  than  in  the  rear  of  the  reflector.  As  the  distance  of  the  observer  from  the  whistle 
was  increased  this  disparity  rapidly  diminished,  and  at  the  distance  of  a  few  hundred 
yards  entirely  disappeared.  The  lyeam  of  sound  had  been  dissipated  and  the  shadow 
had  vanished.  The  effect  of  a  horizontal  sounding-board  10  feet  square,  suspended  over 
the  whistle  to  prevent  the  escape  of  sound  in  a  vertical  direction,  was  inappreciable  at 
the  distance  of  a  quarter  of  a  mile. 

The  employment  of  a  trumpet  with  the  whistle  was  rather  more  successful.  The 
trumpet  was  constructed  of  wood,  in  the  form  of  a  square  pyramid ;  the  lower  base 
being  10'  by  10',  the  upper  base  2'  by  2',  and  the  height  20'.  The  axis  was  horizontal 
and  the  whistle  placed  at  the  smaller  end.  By  this  arrangement  the  increased  power 
of  the  sound  could  be  perceived  at  the  distance  of  a  mile,  the  action  being  similar  to 
that  of  a  speaking-trumpet. 

It  is  probable  that  some  modification  of  this  form  of  whistle  may  be  advantageously 
employed  in  certain  localities,  but  there  is  however  a  disadvantage  attending  the  use 
of  a  Irumpet  with  fog-signals. 

The  sound  from  a  trumpet  not  being  uniformly  distributed,  it  is  difficult  to  estimate 
the  distance  of  the  signal,  or,  as  the  pilots  term  it,  "  to  locate  the  sound."  This  has 
been  observed  in  the  siren  and  Daboll  trumpet.  The  sound  from  these  signals  being 
stronger  on  one  course  than  any  other,  may  be  distinctly  heard  from  a  vessel  when 
crossing  the  axis  of  the  beam  of  sound,  but  as  its  distance  from  this  lino  increases,  the 
sound  appears  fainter  and  more  remote,  although  the  vessel  may  be  approaching  the 
signal. 

From  an  attentive  observation,  during  three  years,  of  the  fog-signals  on  this  coast, 
and  from  the  reports  received  from  captains  and  pilots  of  coasting  vessels,  I  am  con- 
vinced that  in  some  conditions  of  the  atmosphere  the  most  powerful  signals  will  be 
at  times  unreliable. 

Now  it  frequently  occurs  that  a  signal,  which  under  ordinary  circumstances  would 
be  audible  at  the  distance  of  fifteen  miles,  cannot  be  heard  from  a  vessel  at  the  dis- 
tance of  a  single  mile.  This  is  probably  due  to  the  reflection  mentioned  by  Hum- 
boldt. 

The  temperature  of  the  air  over  the  land  where  the  fog-signal  is  located,  being  very 
different  from  that  over  the  sea,  the  sound,  in  passing  from  the  former  to  the  latter, 
undergoes  reflection  at  their  surface  of  contact.  The  correctness  of  this  view  is  ren- 
dered more  probable  by  the  fact  that  when  the  sound  is  thus  impeded  in  the  direction 
of  the  sea,  it  has  been  observed  to  bo  much  stronger  inland. 

When  a  vessel  approaches  a  signal  in  a  fog,  a  difficulty  is  sometimes  experienced  in 
determining  the  position  of  the  signal  by  the  direction  from  which  the  sound  appears 
to  proceed,  the  apparent  and  true  direction  being  entirely  different.  This  is  undoubt- 
edly due  to  the  Detraction  of  sound  passing  through  media  of  different  density. 

Experiments  and  observation  lead  to  the  conclusion  that  these  anomalies  in  the  pen- 
etration and  direction  of  sound  from  fog-signals  are  to  be  attributed  mainly  to  the  want 
of  uniformity  in  the  surrounding  atmosphere,  and  that  snow,  rain,  fog,  and  the  force  and 
direction  of  the  wind,  have  much  less  influence  than  has  generally  been  supposed. 

[In  the  foregoing  I  differ  entirely  in  opinion  from  General  Duane  as 
to  the  cause  of  extinction  of  powerful  sounds  being  due  to  the  unequal 
density  of  the  atmosphere.  The  velocity  of  sound  is  not  at  all  affected 
by  barometric  pressure,  but  if  the  difference  in  pressure  is  caused  by  a 
difference  in  heat,  or  by  the  expansive  power  of  vapor  mingled  with  the 
air,  a  slight  degree  of  obstruction  of  sounds  may  be  observed.  But  this 
effect  we  think  is  entirely  too  minute  to  produce  the  results  noted  by 
General  Duane,  while  we  shall  find  in  the  action  of  the  currents  of  wind 
above  and  below,  a  true  and  sufficient  cause.] 

The  experimental  whistles  were  of  the  following  dimensions,  viz:  2|",  3",  4",  5",  6", 
10",  12",  and  18"  in  diameter.  Those  of  2|",  3",  5",  and  10"  were  fitted,  instead 
of  the  ordinary  bell,  with  long  cylinders,  provided  with  movable  pistons,  so  that 
the  effective  length  of  the  bell  could  be  altered  at  pleasure.  The  pitch  of  the 
blast  was  found  to  vary  with  the  length  of  the  bell,  and  the  power  of  the  whistle 
with  its  diameter.  The  ratio  of  the  power  to  the  diameter  was  not  accurately  ob- 
tained, but  it  is  probable  that  the  extreme  range  of  sound  of  a  whistle  is  proportional 
to  the  square  root  of  its  diameter. 


486  RESEARCHES    IN    SOUND. 

[This  result,  that  the  pitch  varies  with  the  length  of  the  bell,  is  in 
conformity  with  well-established  principles  of  resounding  cavities;  and 
that  the  power  should  increase  with  the  extent  of  the  aerial  reed,  the 
vibrations  of  which  give  motion  to  the  resounding  air  within  the  cavity, 
is  also,  as  we  have  seen,  in  accordance  with  hypothetical  considerations; 
but  as  the  density  of  this  stream  of  steam,  and  consequently  the  rapid- 
ity of  its  vibrations,  depends  upon  the  pressure  of  the  steam  in  the 
boiler,  a  perfect  whistle  should  have  the  capability  of  changing  its  di- 
mensions, not  only  in  relation  to  the  width  of  its  throat,  but  also  in 
regard  to  the  pressure  of  the  steam  in  the  reservoir.] 

The  pitch  giving  the  greatest  range  appears  to  he  at  the  middle  of  the  scale  of  sound. 
It  is  certain  that  a  good  result  cannot  be  obtained  from  either  a  very  shrill  or  a  bass 
note.  This  remark  is  applicable  to  all  varieties  of  signal. 

The  10"  and  12"  whistles  are  recommended  for  ordinary  use.  The  18"  whistle  is 
more  powerful,  but  the  increase  of  power  bears  too  small  a  proportion  to  that  of  the 
expenditure  of  fuel  to  render  its  employment  generally  advisable.  The  best  results 
were  obtained  by  giving  the  whistle  the  following  proportions :  The  diameter  of  the 
bell  equaling  two- thirds  of  its  length,  and  the  set  of  the  bell,  i.  e.,  the  vertical  dis- 
tance of  the  lower  edge  above  the  cup,  the  one-third  to  one-fourth  of  the  diameter  for 
a  pressure  of  50  to  60  pounds  of  steam 

A  bell,  whether  operated  by  hand  or  by  machinery,  cannot  be  considered  an  efficient 
fog-signal  on  the  sea-coast.  In  calm  weather  it  cannot  be  heard  half  the  time  at  a 
greater  distance  than  one  mile,  while  in  rough  weather  the  noise  of  the  surf  will 
drown  its  sound  to  seaward  altogether. 

Ou  approaching  a  station  I  have  frequently  seen  the  bell  rung  violently  by  the 
keeper,  without  being  able  to  hear  the  sound  until  I  had  landed. 

Nevertheless,  all  important  stations  should  be  provided  with  bells,  as  there  are  occa- 
sions when  they  may  serve  a  useful  purpose,  but  it  should  be  well  understood  by  mari- 
ners that  they  must  not  expect  always  to  hear  the  bells  as  a  matter  of  course. 

Bells  should  not  be  omitted  at  stations  furnished  with  steam  fog-signals,  especially 
when  the  latter  are  not  in  duplicate,  and  mariners  should  be  warned  that  the  bell  will 
be  sounded  when  the  regular  signal  is  disabled. 

It  has  been  observed  that  a  bell  rung  by  hand  can  be  heard  further  than  when 
sounded  by  machinery,  and  many  of  the  steamboat  companies  on  this  coast  pay  the 
keepers  of  bells  rung  by  clock-work  to  ring  them  by  hand  when  the  boats  of  their 
line  are  expected  to  pas's. 

[We  think  the  difference  in  the  effect  of  ringing  of  bells  by  hand  or 
by  machinery  is  so  slight  as  to  be  inappreciable  except  at  a  short  dis- 
tance. It  is  true,  as  I  have  before  observed,  that  the  sound  is  louder 
when  the  mouth  of  the  bell  is  directed  toward  the  hearer  than  when  the 
edge  is  so  directed,  but  on  account  of  the  spreading  of  this  sound  the 
effect  is  lost  in  a  small  distance,  and  indeed  in  one  light-house  the  bell 
is  permanently  placed  with  the  axis  of  its  mouth  directed  horizontally, 
and  in  this  position,  if  the  bell  Were  struck  interiorly  with  a  hammer, 
which  would  give  it  a  larger  vibration  than  when  struck  exteriorly,  I 
doubt  whether  any  difference  would  be  observed  between  the  two  meth- 
ods of  ringing ;  and  if  any  existed  it  would  probably  be  in  favor  of  the 
fixed  bell  rung  by  machinery.] 

On  rivers,  narrow  channels,  and  lakes,  where  the  difficulty  from  the  noise  of  the  surf 
does  not  exist,  this  species  of  signal  may  be  used  to  advantage,  as  its  maintenance 
requires  but  a  small  expenditure  of  either  money  or  labor,  and  by  a  proper  arrange- 
ment of  the  machinery  the  intervals  between  the  strokes  of  the  bell  may  be  so  regu- 
lated as  to  avoid  the  danger  of  confounding  the  signals,  however  near  together. 

Although  a  bell  may  be  heard  better  when  sounded  by  hand  than  by  clock-work,  yet 
in  thoroughfares  where  the  signal  must  be  kept'in  constant  operation  during  the  en- 
tire continuance  of  a  fog,  it  would  be  impracticable  to  make  use  of  the  former  method, 
and  recourse  must  be  had  to  machinery. 


RESEARCHES    IN    SOUND.  487 

In  arranging  the  signal  the  bell  and  machinery  rnnst  be  placed  as  low  as  possible, 
as  the  sound  is  heard  much  more  plainly  on  the  water  when  the  bell  is  near  its  sur- 
face, and  also  as  the  machinery,  when  thus  situated,  is  steadier  and  more  readily  ac- 
cessible. 

Particulars  as  to  the  siren. — The  boiler  of  a  second-class  apparatus  is  12  feet  long,  42 
inches  in  diameter,  and  has  300  feet  heating-surface.  The  dome  is  2  feet  in  diameter 
and  3  feet  high. 

The  cylinder  of  the  engine  is  4  inches  in  diameter  and  6  inches  stroke.  The  prolon- 
gation of  the  piston-rod  forms  the  plunger  of  the  feed-pump.  The  main  shaft  carries 
three  pulleys,  the  larger  driving  the  siren-spindle;  the  second,  the  worm  and  screw 
gear ;  and  the  third,  the  governor. 

In  the  worm-gear  the  wheel  makes  two  revolutions  per  minute,  and  is  provided  with 
a  cam,  which,  acting  on  a  lever,  opens  the  valve,  admitting  steam  through  the  siren  - 
disks.  The  cam  has  such  a  length  as  to  hold  the  valve  open  for  about  seven  seconds. 
A  counter-weight  closes  the  valve  as  soon  as  the  lever  is  released  by  the  cam. 

The  siren  itself  consists  of  a  cylindrical  steam-chest,  closed  at  one  end  by  a  perfor- 
ated brass  plate.  The  perforations  are  twelve  in  number,  equidistant  from  each  other, 
and  arranged  on  the  circumference  of  a  circle,  whose  center  is  in  the  axis  of  the  cyl- 
inder. The  other  end  is  closed  by  a  cast-iron  head.  The  heads  are  connected  by  a 
brass  pipe,  through  which  the  spindle  passes. 

The  perforated  head  is  covered  on  the  exterior  by  a  brass  disk,  attached  to  the 
spindle,  having  twelve  rectangular  notches  corresponding  to  the  apertures  on  the  for- 
mer, and  so  arranged  that  by  its  revolution  these  apertures  are  simultaneously  opened 
and  closed.  The  spindle  is  driven  by  a  belt  from  the  large  pulley  on  the  main  shaft. 
This  shaft  makes  180  revolutions  per  minute;  the  epindle,  1,620;  and  as  there  are  12 
apertures  in  the  disks,  from  each  there  will  issue  jets  of  steam  at  the  rate  of  19,440 
per  minute.  The  sound  produced  by  these  impulses  may  be  rendered  more  or  less 
acute  by  increasing  or  diminishing  the  velocity  of  revolution. 

The  valve  and  valve-seat  are  disks  similar  to  those  already  described,  having  how- 
ever four  openings  instead  of  twelve.  The  valve  revolves  on  the  brass  tube  inclosing 
the  siren-spindle,  "and  is  worked  by  a  bevel  gear.  The  trumpet  is  of  cast-iron. 

The  Daboll  trumpet. — The  apparatus  used  in  the  foregoing  experiments  is  a  second- 
class  trumpet,  operated  by  an  Ericsson  caloric-engine.  The  air-pump  is  single-acMng. 
Its  cylinder  is  12"  in  diameter  by  12"  stroke.  The  engine  makes  forty  strokes  per  min- 
ute. There  is  a  screw-thread  raised  on  the  main  shaft,  which,  acting  on  a  wheel, 
drives  a  bevel  gear,  giving  motion  to  a  cam- wheel.  The  latter  makes  one  revolution 
in  two  minutes,  and  is  furnished  with  three  equidistant  cams.  These  cams,  pressing 
on  the  valve-lever,  throw  the  valve  open  once  in  forty  seconds,  admitting  the  com- 
pressed air  through  the  reed-chest  into  the  trumpet. 

The  quantity  of  air  forced  into  the  tank  should  be  in  excess  of  that  needed  for  the 
trumpet,  the  surplus  being  allowed  to  escape  through  a  delicate  safety-valve.  This  is 
necessary  to  provide  against  a  deficiency  in  case  of  leakage,  and  also  to  allow  the 
pressure  of  air  to  be  regulated  to  accommodate  the  reed.  Each  reed  requiring  a  differ- 
ent pressure,  it  is  necessary  to  alter  the  pressure  of  the  valve-spring  whenever  a  reed 
is  changed. 

The  first-class  trumpet  differs  only  in  size  from  that  described. 

The  caloric-engine  for  the  first  class  has  a  30"  cylinder.  The  air-pump  is  16^"  by 
15"  stroke. 

The  steam-whistle. — The  boiler  of  this  machine  is  that  of  the  siren.  On  the  forward 
part  of  the  boiler  the  bed-plate  of  a  small  engine  is  secured  by  two  cast-iron  brackets. 
The  cylinder  of  this  engine  is  4"  by  9".  The  fly-wheel  shaft  carries  an  eccentric, 
which,  acting  through  a  rod  and  pawl  on  a  ratchet-wheel,  gives  the  required  motion 
to  the  cam-wheel  shaft. 

The  cam-wheel,  which  makes  one  revolution  per  minute,  is  provided  with  one  or 
more  cams,  depending  on  the  number  of  blasts  to  be  given  in  a  minute  ;  the  length  of 
the  blast  being  regulated  by  that  of  the  cams. 

The  valve  for  admitting  the  steam  into  the  whistle  is  a  balance-valve,  the  diame- 
ters of  the  two  disks  being  respectively  3£"  and  2f ",  which  difference  is  sufficient  to 
cause  the  pressure  of  steam  to  close  the  valve  tight  without  requiring  too  great  a  force 
to  open  it.  The  valve  is  worked  by  a  stem  attached  to  the  rocker-shaft  at  the  lower 
part  of  the  steam -pipe.  This  shaft  passes  through  a  stuffing-box  in  the  steam-pipe, 
and  is  provided  with  a  collar  which  the  pressure  of  the  steam  forces  against  the  inte- 
rior boss  on  the  pipe,  thus  making  the  joint  steam-tight.  The  exterior  arm  on  this 
rocker-shaft,  as  well  as  that  on  the  engine,  is  perforated  in  such  a  manner  as  to  allow 
the  throw  of  the  valve  to  be  adjusted. 


488  RESEARCHES    IN   SOUND. 

In  the  comments  we  have  made  on  the  report  of  General  Duane,  the 
intention  was  not  in  the  least  to  disparage  the  value  of  his  results,  which 
can  scarcely  be  too  highly  appreciated ;  but  inasmuch  as  the  true  explana- 
tion of  the  phenomena  he  has  observed  has  an  important  bearing  on  the 
location  of  fog-signals  and  on  their  general  application  as  aids  to  naviga- 
tion, and  are  as  well  of  great  interest  to  the  physicist,  who  values  every 
addition  to  theoretical  as  well  as  practical  knowledge,  we  have  not  only 
thought  the  remarks  we  have  offered  necessary,  but  also  that  special 
investigations  should  be  made  to  ascertain  more  definitely  the  condi- 
tions under  which  the  abnormal  phenomena  the  general  has  described 
occur,  and  to  assign,  if  possible,  a  more  definite  and  efficient  cause  than 
those  to  which  he  has  attributed  them. 

We  have,  therefore,  given  much  thought  to  the  subject,  and  since  the 
date  of  General  Duane's  report,  have  embraced  every  opportunity  which 
occurred  for  making  observations  in  regard  to  them.  The  first  step  we 
made  toward  obtaining  a  clew  to  the  explanation  of  the  phenomena  in 
question  resulted  from  observatiops  at  New  Haven,  namely :  1st,  the 
tendency  of  sound  to  spread  laterally  into  its  shadow  5  2d,  the  fact  that 
a  sound  is  frequently  borne  in  an  opposite  direction  to  the  wind  at  the 
surface  by  an  upper  current ;  and  3d,  that  a  sound  moving  against  a  wind 
is  heard  better  at  a  higher  elevation.  The  first  point  to  consider  is  in 
what  manner  the  wind  affects  sound.  That  it  is  in  some  way  connected 
with  the  distance  to  which  sound  can  be  heard  is  incontestably  settled  by 
general  observation.  At  first  sight,  the  explanation  of  this  might  seem 
to  be  very  simple,  namely,  that  the  sound  is  borne  on  in  the  one  direc- 
tion and  retarded  in  the  other  by  the  motion  of  the  wind.  But  this  ex- 
planation, satisfactory  as  it  might  appear,  cannot  be  true.  Sound  moves 
at  the  rate  of  about  780  miles  an  hour,  and  therefore,  on  the  above  sup- 
position, a  wind  of  7.8  miles  per  hour  could  neither  retard  nor  accele- 
rate its  velocity  more  than  one  per  cent.,  an  amount  inappreciable  to 
ordinary  observation ;  whereas  we  know  that  a  wind  of  the  velocity  we 
have  mentioned  is  frequently  accompanied  with  a  reduction  of  the  pene- 
trating power  of  sound  of  more  than  50  per  cent. 

The  explanation  of  this  phenomenon,  as  suggested  by  the  hypothesis  ot 
Professor  Stokes,  is  founded  on  the  fact  that  in  the  case  of  a  deep  current 
of  air  the  lower  stratum,  or  that  next  the  earth,  is  more  retarded  by  fric- 
tion than  the  one  immediately  above,  and  this  again  than  the  one  above  it, 
and  so  on.  The  effect  of  this  diminution  of  velocity  as  we  descend  toward 
the  earth  is,  in  the  case  of  sound  moving  with  the  current,  to  carry  the 
upper  part  of  the  sound-waves  more  rapidly  forward  than  the  lower  parts, 
thus  causing  them  to  incline  toward  the  earth,  or,  in  other  words,  to  be 
thrown  down  upon  the  ear  of  the  observer.  When  the  sound  is  in  a 
contrary  direction  to  the  current,  an  opposite  effect  is  produced, — the 
upper  portion  of  the  sound-waves  is  more  retarded  than  the  lower,  which 
advancing  more  rapidly,  in  consequence  inclines  the  waves  upward  and 
directs  tjiem  above  the  head  of  the  observer.  To  render  this  more  clear, 


RESEARCHES    IN    SOUND.  489 

let  us  recall  the  nature  of  a  beam  of  sound,  in  still  air,  projected  in 
a  horizontal  direction.  It  consists  of  a  series  of  concentric  waves  per- 
pendicular to  the  direction  of  the  beam,  like  the  palings  of  a  fence. 
Now,  if  the  upper  part  of  the  waves  has  a  slightly  greater  velocity 
than  the  lower,  the  beam  will  be  bent  downward  in  a  manner  some- 
what analogous  to  that  of  a  ray  of  light  in  proceeding  from  a  rarer  to  a 
denser  medium.  The  effect  of  this  deformation  of  the  wave  will  be 
cumulative  from  the  sound-center  onward,  and  hence,  although  the  ve- 
locity of  the  wind  may  have  no  perceptible  effect  on  the  velocity  of  sound, 
yet  this  bending  of  the  wave  being  continuous  throughout  its  entire 
course,  a  marked  effect  must  be  produced. 

A  precisely  similar  effect  will  be  the  result,  but  perhaps  in  a  consid- 
erably greater  degree,  in  case  an  upper  current  is  moving  in  an  opposite 
direction  to  the  lower,  when  the  latter  is  adverse  to  the  sound,  and  in 
this  we  have  a  logical  explanation  of  the  phenomenon  observed  by 
General  Duane,  in  which  a  fog-signal  is  only  heard  during  the  occurrence 
of  a  northeast  snow-storm.  Certainly  this  phenomenon  cannot  be  ex- 
plained by  any  peculiarity  of  the  atmosphere  as  to  variability  of  density, 
or  of  the  amount  of  vapor  which  it  may  contain. 

The  first  phenomenon  of  the  class  mentioned  by  General  Duane,  which 
I  had  the  good  fortune  to  witness  was  in  company  with  Sir  Frederick 
Arrow  and  Captain  Webb,  of  the  Trinity  House,  London,  in  their  visit 
to  this  country  in  1872.  At  the  distance  of  two  or  three  miles  from  an 
island  in  the  harbor  of  Portland,  Maine,  on  which  a  fog-signal  was 
placed,  the  sound,  which  had  been  distinctly  heard,  was  lost  on  ap- 
proaching the  island  for  nearly  a  mile,  and  slightly  regained  at  a  less 
distance.  On  examining  the  position  of  the  fog-signal,  which  was  sit- 
uated on  the  farther  side  of  the  island  from  the  steamer,  we  found  it 
placed  immediately  in  front  of  a  large  house  with  rising  ground  in  the 
rear,  which  caused  a  sound-shadow,  into  which,  on  account  of  the  late- 
ral divergence  of  the  rays,  the  sound  was  projected  at  a  distance,  but 
not  in  the  immediate  vicinity  of  $he  island.  In  the  same  year  I  made 
an  excursion  in  one  of  the  light-house  steamers,  with  Captain  Selfridge, 
to  an  island  on  the  coast  of  Maine,  at  which  abnormal  phenomena  were 
said  to  have  been  observed,  but  on  this  occasion  no  variation  of  the 
sound  was  noted,  except  that  which  was  directly  attributable  to  the 
wind,  the  signal  being  heard  much  farther  in  one  direction  than  in  the 
opposite. 


490  RESEARCHES   IN   SOUND. 

PAET  II.— EEMAEKS  ON  SOME  ABNOEMAL  PHENOMENA  OF 

SOUND.* 

The  communication  which  I  propose  to  make  this  evening  is  brought 
forward  at  this  time  especially  on  account  of  the  presence  of  Dr.  Tyn- 
dall,  he  being  connected  with  the  light-house  system  of  Great  Britain, 
while  the  facts  I  have  to  state  are  connected  with  the  light-house  service 
of  the  United  States,  and  must  therefore  be  of  interest  to  our  distin- 
guished visitor.  The  facts  I  have  to  present  form  part  of  a  general  re- 
port to  be  published  by  the  United  States  Light-House  Board. 

The  Light-House  Board  of  the  United  States  has  from  its  first  estab- 
lishment aimed  not  only  to  furnish  our  sea-coast  with  all  the  aids  to 
navigation  that  have  been  suggested  by  the  experience  of  other  countries 
and  to  adopt  the  latest  improvements,  but  also  to  enrich  the  light-house 
service  with  the  results  of  new  investigations  and  new  devices  for  the 
improvement  of  its  efficiency,  or,  in  other  words,  to  add  its  share  to  the 
advance  of  a  system  which  pertains  to  the  wants  of  the  highest  civiliza- 
tion. 

Among  the  obstructions  to  navigation  none  are  more  serious,  especially 
on  the  American  coast,  than  those  caused  by  fogs. 

Fog,  as  it  is  well  known,  is  due  to  the  mingling  of  warmer  air  sur- 
charged with  moisture  with  colder  air,  and  nowhere  on  the  surface  of 
the  earth  do  more  favorable  conditions  exist  for  producing  fogs 
than  on  both  our  Atlantic  and  Pacific  coasts.  On  the  Atlantic  the 
cold  stream  of  water  from  the  polar  regions  in  its  passage  southward, 
on  account  of  the  rotation  of  the  earth,  passes  close  along  our  eastern 
coast  from  one  extremity  to  the  other,  and  parallel  to  this  but  opposite 
in  direction,  for  a  considerable  distance  is  the  great  current  of  warm 
water  known  as  the  Gulf  Stream.  Above  the  latter  the  air  is  constantly 
surcharged  with  moisture,  and  consequently  whenever  light  winds  blow 
from  the  latter  across  the  former,  the  vapor  is  condensed  into  fog,  and 
since  in  summer  along  our  eastern  coast  the  southerly  wind  prevails, 
we  have  during  July,  August,  and  September,  especially  on  the  coast 
of  Maine,  an  almost  continuous  prevalence  of  fogs  so  dense  that  distant 
vision  is  entirely  obstructed. 

On  the  western  coast  the  great  current  of  the  Pacific,  after  having 
been  cooled  in  the  northern  regions,  in  its  passage  southward  gives  rise 
to  cold  and  warm  water  in  juxtaposition,  or,  in  other  words,  a  current  of 
the  former  through  the  latter,  and  hence  whenever  a  wind  blows  across 
the  current  of  cold  water,  a  fog  is  produced. 

From  the  foregokig  statement  it  is  evident  that  among  the  aids  to 
navigation  fog-signals  are  almost  as  important  as  light-houses.  The 
application  however  of  the  science  of  acoustics  to  the  former  is  far  less 
advanced  than  is  that  of  optics  to  the  latter.  Indeed,  attempts  have 
been  made  to  apply  lights  of  superior  penetrating  power,  as  the  electric 

*  Made  before  the  "  Philosophical  Society  of  Washington,"  December  11,  1872. 


RESEARCHES   IN   SOUND.  491 

and  calcium  lights,  to  supersede  the  imperfect  fog-signals  in  use.  When 
however  we  consider  the  fact  that  the  absorptive  power  of  a  stratum  of 
cloud,  which  is  but  a  lighter  fog,  of  not  more  than  two  or  three  miles  in 
thickness,  is  sufficient  to  obscure  the  image  of  the  sun,  the  intensity  of 
the  light  of  which  is  greater  than  that  of  any  artificial  light,  it  must  be 
evident  that  optical  means  are  insufficient  for  obviating  the  difficulty  in 
question. 

The  great  extent  of  the  portions  of  the  coast  of  the  United  States 
which  are  subject  to  fogs  renders  the  investigation  of  the  subject  of  fog- 
signals  one  of  the  most  important  duties  of  the  Light-House  Board. 

In  studying  this  subject  it  becomes  a  question  of  importance  to  ascer- 
tain whether  waves  of  sound,  like  those  of  light,  are  absorbed  or  stifled 
by  fog 5  on  this  point  however,  observers  disagree.  At  first  sight,  from 
the  very  striking  analogy  which  exists  in  many  respects  between  light 
and  sound,  the  opinion  has  largely  prevailed  that  sound  is  impeded  by  fog. 
But  those  who  have  not  been  influenced  by  this  analogy  have  in  some 
instances  adopted  the  opposite  opinion — that  sound  is  better  heard  during 
a  fog  than  in  clear  weather.  To  settle  this  question  definitely  the  Light- 
House  Board  have  directed  that  at  two  light-houses  on  the  route  from 
Boston  to  Saint  John  the  fog-signals  shall  be  sounded  every  day  on 
which  the  steamboats  from  these  ports  pass  the  station,  both  in  clear 
and  foggy  weather,  the  pilots  on  board  these  vessels  having,  for  a  small 
gratuity,  engaged  to  note  the  actual  distance  of  the  boat  when  the  sound 
is  first  heard  on  approaching  the  signal  and  is  last  heard  on  receding 
from  it.  The  boats  above  mentioned  estimate  their  distance  with  con- 
siderable precision  by  the  number  of  revolutions  of  the  paddle-wheel  as 
recorded  by  the  indicator  of  the  engine,  and  it  is  hoped  by  this  means 
to  definitely  decide  the  point  in  question.  We  think  it  probable  that 
fog  does  somewhat  diminish  the  penetrating  power  of  sound,  or  in 
other  words,  produce  an  effect  analogous  to  that  on  the  propagation 
of  light.  But  when  we  consider  the  extreme  minuteness  of  the  parti- 
cles of  water  constituting  the  fog  as  compared  with  the  magnitude  of 
the  waves  of  sound,  the  analogy  does  not  hold  except  in  so  small  a  de- 
gree as  to  be  of  no  practical  importance,  or,  in  other  words,  the  exist- 
ence of  a  fog  is  a  true,  but,  we  think,  a  wholly  insufficient  cause  of  di- 
minution of  sound,  which  view  is  borne  out  by  the  great  distance  at 
which  our  signals  are  heard  during  a  dense  fog. 

Another  cause,  which  without  doubt  is  a  true  one,  of  the  diminution 
of  the  penetrating  power  of  sound  is  the  varying  density  of  the  atmos- 
phere, from  heat  and  moisture,  in  long  distances.  The  effect  of  this, 
however,  Avould  apparently  be  to  slightly  distort  the  wave  of  sound 
rather  than  to  obliterate  it.  However  this  may  be,  we  think,  from  all 
the  observations  we  have  made,  the  effect  is  small  in  comparison  with 
another  cause,  viz,  that  of  the  influence  of  wind.  During  a  residence  of 
several  weeks  at  the  sea-shore,  the  variation  in  intensity  of  the  sound 


492  RESEARCHES   IN   SOUND. 

of  the  breakers  at  a  distance  of  about  a  mile  in  no  case  appeared  to  be 
coincident  with  the  variations  of  an  aneroid  barometer  or  a  thermome- 
ter, but  in  every  instance  it  was  affected  by  the  direction  of  the  wind. 

The  variation  in  the  distinctness  of  the  sound  of  a  distant  instrument 
as  depending  on  the  direction  of  the  wind  is  so  marked  that  we  are  war- 
ranted in  considering  it  the  principal  cause  of  the  inefficiency  in  certain 
cases  of  the  most  powerful  fog-signals.  The  effect  of  the  wind  is  usu- 
ally attributed,  without  due  consideration,  to  the  motion  of  the  body  of 
air  between  the  hearer  and  the  sounding  instrument ;  in  the  case  of  its 
coming  towards  him  it  is  supposed  that  the  velocity  of  the  sound  is  re- 
inforced by  the  motion  of  the  air,  and  when  in  the  opposite  direction 
that  it  is  retarded  in  an  equal  degree.  A  little  reflection,  however,  will 
show  that  this  cannot  be  the  cause  of  the  phenomenon  in  question,  since 
the  velocity  of  sound  is  so  vastly  greater  than  that  of  any  ordinary 
wind  that  the  latter  can  only  impede  the  progress  of  the  former  by  a 
very  small  percentage  of  the  whole.  Professor  Stokes,  of  Cambridge 
University,  England,  has  offered  a  very  ingenious  hypothetical  explana- 
tion of  wind  on  sound,  which  we  think  has  an  important  practical  bear- 
ing, especially  in  directing  the  line  of  research  and  subsequent  applica- 
tion of  principles. 

His  explanation  rests  upon  the  fact  that  during  the  passage  of  a  wind 
between  the  observer  and  the  sounding  instrument  the  velocity  of  this 
will  be  more  retarded  at  the  surface  of  the  earth  on  account  of  friction 
and  other  obstacles,  and  that  the  velocity  of  the  stratum  immediately 
above  will  be  retarded  by  that  below,  and  so  on,  the  obstruction  being 
lessened  as  we  ascend  through  the  strata.  From  this  it  follows  that  the 
sound  wave  will  be  deformed  and  the  direction  of  its  normal  changed. 
Suppose,  for  example,  that  the  wind  is  blowing  directly  from  the  olS- 
server.  In  this  case  the  retardation  of  the  sound  wave  will  be  greater 
above  than  below,  and  the  upper  part  of  the  wave-front  will  be  thrown 
backwards  so  that  the  axis  of  the  phonic  ray  will  be  deflected  upwards, 
and  over  the  head  of  the  observer.  If,  on  the  other  hand,  a  deep  river 
of  wind  (so  to  speak)  is  blowing  directly  towards  the  observer,  the  upper 
part  of  the  front  of  the  wave  will  be  inclined  down  and  towards  him, 
concentrating  the  sound  along  the  surface  of  the  earth. 

The  science  of  acoustics  in  regard  to  the  phenomena  of  sound  as  ex- 
hibited in  limited  spaces  has  been  developed  with  signal  success.  The 
laws  of  its  production,  propagation,  reflection,  and  refraction  have  been 
determined  with  much  precision,  so  that  we  are  enabled  in  most  cases 
to  explain,  predict,  and  control  the  phenomena  exhibited  under  given 
conditions.  But  in  case  of  loud  sounds  and  those  which  are  propagated 
to  a  great  distance,  such  as  are  to  be  employed  as  fog-signals,  consider- 
able obscurity  still  exists.  As  an  illustration  of  this  I  may  mention  the 
frequent  occurrence  of  apparently  abnormal  phenomena.  General  War- 
ren informs  me  that  at  the  battle  of  Seven  Pines,  in  June,  1862,  near 
Bichmond,— General  Johnston,  of  the  Confederate  army,  was  within  three 


RESEARCHES   IN   SOUND.  493 

miles  of  the  scene  of  action  with  a  force  intended  to  attack  the  flank  of 
the  Northern  forces,  and  although  listening  attentively  for  the  sound  of 
the  commencement  of  the  engagement,  the  battle,  which  was  a  severe 
one,  and  lasting  about  three  hours,  ended  without  his  having  heard  a 
single  gun.  (See  Johnston's  report.)  Another  case  of  a  similar  kind 
occurred  to  General  McClellan  at  the  battle  of  Games'  Mills,  June  277 
1862,  also  near  Richmond.  Although  a  sharp  engagement  was  progress- 
ing within  three  or  four  miles  for  four  or  five  hours,  the  general  and  his 
staff  were  unaware  of  its  occurrence,  and  when  their  attention  was 
called  to  some  feeble  sound  they  had  no  idea  that  it  was  from  anything 
more  than  a  skirmish  of  little  importance.  (See  Eeport  of  the  Commit- 
tee on  the  Conduct  of  the  War.)  A  third  and  perhaps  still  more  re- 
markable instance  is  given  in  a  skirmish  between  a  part  of  the  Second 
Corps  under  General  Warren  and  a  force  of  the  enemy.  In  this  case 
the  sound  of  the  tiring  was  heard  more  distinctly  at  General  Meade's 
headquarters  than  it  was  at  the  headquarters  of  the  Second  Corps  itself, 
although  the  latter  was  about  midway  between  the  former  and  the  point 
of  conflict.  Indeed  the  sound  appeared  so  near  General  Meade's  camp 
that  the  impression  was  made  that  the  enemy  had  gotten  between  it  and 
General  Warren's  command.  In  fact  so  many  instances  occurred  of  wrong 
impressions  as  to  direction  and  distance  derived  from  the  sound  of  guns 
that  little  reliance  came  to  be  placed  on  these  indications. 

In  the  report  of  a  series  of  experiments  made  under  the  direction  of 
the  Light-House  Board  by  General  Duane  of  the  Engineer  Corps  is  the 
following  remark :  "  The  most  perplexing  difficulty  arises  from  the  fact 
that  the  fog-signal  often  appears  to  be  surrounded  by  a  belt  varying  in 
radius  from  one  to  one  and  a  half  miles.  Thus  in  moving  directly  from 
a  station  the  sound  is  audible  for  the  distance  of  a  mile,  is  then  lost  for 
about  the  same  distance,  after  which  it  is  again  distinctly  heard  for  a 
long  time." 

Again,  in  a  series  of  experiments  at  which  Sir  Frederick  Arrow  and  Cap- 
tain Webb,  of  the  Trinity  Board,  assisted,  it  was  found  that  in  passing  in 
the  rear  of  the  opposite  side  of  an  island  in  front  of  which  a  fog-signal 
was  placed,  the  sound  entirely  disappeared,  but  by  going  further  off  to 
the  distance  of  two  or  three  miles  it  reappeared  in  full  force,  even  with 
a  large  island  intervening.  Again,  from  the  experiments  made  under  the 
immediate  direction  of  the  present  chairman  of  the  Light-House  Board, 
with  the  assistance  of  Admiral  Powell  and  Mr.  Lederle,  the  light- 
house engineer,  and  also  from  separate  experiments  made  by  General 
Duane,  it  appears  that  while  a  reflector,  in  the  focus  of  which  a  steam 
whistle  or  ordinary  bell  is  placed,  reinforces  the  sound  for  a  short  dis- 
tance, it  produces  little  or  no  effect  at  the  distance  of  two  or  three  miles, 
and,  indeed,  the  instrument  can  be  -as  well  heard  in  still  air  at  the  dis- 
tance of  four  or  five  miles  in  the  line  of  the  axis  of  the  reflector,  whether 
the  ear  be  placed  before  or  behind  it.  From  these  results  we  would 
infer  that  the  lateral  divergency  of  sound,  or  its  tendency  to  spread  lat- 


494  RESEARCHES    IN   SOUND. 

erally  as  it  passes  from  its  source,  is  much  greater  than  has  been  sup* 
posed  from  experiments  on  a  small  scale.  The  idea  we  wish  to  convey 
by  this  is  that  a  beam  of  sound  issuing  through  an  orifice,  although  at 
first  proceeding  like  a  beam  of  light  in  paralled  rays,  soon  begins  to 
diverge  and  spread  out  into  a  cone,  and  at  a  sufficient  distance  may  in- 
clude even  the  entire  horizon. 

We  may  mention  also  in  this  connection  that  from  the  general  fact 
expressed  by  the  divergence  of  the  rays  of  sound,  the  application  of 
reflection  as  a  means  of  reinforcing  sound  must  in  a  considerable  degree 
of  necessity  be  a  failure. 

By  the  application  of  the  principle  we  have  stated  and  the  effect  of 
the  wind  in  connection  with  the  peculiarities  of  the  topography  of  a 
region  and  the  position  of  the  sounding  body,  we  think  that  not  only 
may  most  of  the  phenomena  we  have  just  mentioned  be  accounted  for, 
but  also  that  other  abnormal  effects  may  be  anticipated. 

In  critically  examining  the  position  of  the  sounding  body  in  the  experi- 
ment we  have  mentioned,  in  which  Sir  Frederick  Arrow  and  Captain  Webb 
assisted,  it  was  found  that  the  signal  was  placed  on  the  side,  of  a  bank 
with  a  large  house  directly  in  the  rear,  thereof  of  which  tended  to  deflect 
the  sound  upwards  so  as  to  produce  in  the  rear  a  shadow,  but  on  account 
of  the  divergency  of  the  beam  this  shadow  vanished  at  the  distance  of 
a  mile  and  a  half  or  two  miles,  and  at  the  distance  of,  say,  three  miles 
the  sound  of  the  instrument  was  distinctly  heard.  I  doubt  not  that,  on 
examination,  all  the  cases  mentioned  by  General  Duane,  with  one  ex- 
ception, might  be  referred  to  the  same  principle,  the  exception  being 
expressed  in  the  following  remarkable  statement  in  his  report  to  the 
Light-House  Board:  " The  fog-signals  have  frequently  been  heard  at  a 
distance  of  twenty  miles  and  as  frequently  cannot  be  heard  at  the  distance 
of  two  miles,  and  with  no  perceptible  difference  in  the  state  of  the  atmos- 
phere. The  signal  is  often  heard  at  a  greater  distance  in  one  direction, 
while  in  another  it  will  be  scarcely  audible  at  the  distance  of  a  mile. 
For  example,  the  whistle  at  Cape  Elizabeth  can  always  be  distinctly  heard 
in  Portland — a  distance  of  nine  miles — during  a  heavy  northeast  snow- 
storm, the  wind  blowing  a  gale  nearly  from  Portland  towards  the  whistle." 

This  is  so  abnormal  a  case,  and  so  contrary  to  generally  received  opinion, 
that  I  hesitated  to  have  it  published  under  the  authority  of  the  board 
until  it  could  be  verified  and  more  thoroughly  examined.  In  all  the 
observations  that  have  been  made  under  my  immediate  supervision,  the 
sound  has  always  been  heard  farther  with  the  wind  than  against  it.  It 
would  appear,  therefore,  from  all  the  observations  that  the  normal  effect 
of  the  wind  is  to  diminish  the  sound  in  blowing  directly  against  it. 

There  is  however  a  meteorological  condition  of  the  atmosphere  dur- 
ing a  northeast  storm  on  our  coast  which  appears  to  me  to  have  a 
direct  bearing  on  the  phenomenon  in  question.  It  is  this :  that  while  a 
violent  wind  is  blowing  from  the  northeast  into  the  interior  of  the  country, 
a  wind  of  equal  intensity  is  blowing  in  an  opposite  direction  at  an  ele- 


RESEARCHES    IN    SOUND.  495 

vation  of  a  mile  or  two.    This  is  shown  by  the  rapid  east  wardly  motion 
of  the  upper  clouds  as  occasionally  seen  through  breaks  in  the  lower. 

As  a  further  illustration  of  this  principle  I  may  mention  that  on  one 
occasion  (in  1855)  I  started,  on  my  way  to  Boston  from  Albany,  in  the  morn- 
ing of  a  clear  day,  with  a  westerly  wind.  The  weather  continued  clear 
and  pleasant  until  after  passing  the  Connecticut  Eiver,  and  until  within 
fifty  miles  of  Bosfon.  We  then  encountered  a  storm  of  wind  and  rain 
which  continued  until  we  reached  the  city.  On  inquiry  I  learned  that 
the  storm  had  commenced  in  Boston  the  evening  before,  and,  although 
the  wind  had  been  blowing  violently  towards  Albany  for  twenty  hours, 
it  had  not  reached  inwardly  more  than  fifty  miles.  At  this  point  it  met 
the  west  wind  and  was  turned  back  above  in  almost  a  parallel  current. 
This  is  the  general  character  of  northeast  storms  along  our  coast,  as 
shown  by  Mr.  Espy,  and  is  directly  applicable  to  the  phenomenon  men- 
tioned by  General  Duane,  and  which,  from  the  frequency  with  whicli  he 
has  witnessed  the  occurrence,  we  must  accept  as  a  fact,  though  by  no 
means  a  general  one  applicable  to  all  stations.  While  a  violent  wind  was 
blowing  towards  his  place  of  observation  from  Cape  Elizabeth,  at  the 
surface  of  the  earth,  a  parallel  current  of  air  was  flowing  above  with 
equal  or  greater  velocity  in  the  opposite  direction.  The  effect  of  the 
latter  would  be  to  increase  the  velocity  of  the  upper  part  of  the  wave 
of  sound,  and  of  the  former  to  diminish  it ;  the  result  of  the  two  being 
to  incline  the  front  of  the  wave  of  sound  towards  the  observer,  or  to 
throw  it  down  towards  the  earth,  thus  rendering  the  distant  signal  audi- 
ble under  these  conditions  when  otherwise  it  could  not  be  heard.  I  think 
it  is  probable  that  the  same  principle  applies  in  other  cases  to  the  abnor- 
mal propagation  of  sound. 

For  the  production  of  a  sound  of  sufficient  power  to  serve  as  a  fog- 
signal,  bells,  gongs,  &c.,  are  too  feeble  except  in  special  cases  where  the 
warning  required  is  to  be  heard  only  at  a  small  distance.  After  much 
experience,  the  Light-House  Board  has  adopted,  for  first-class  signals, 
instruments  actuated  by  steam  or  hot-air  engines,  and  such  only  as  de- 
pend upon  the  principle  of  resonance,  or  the  enforcement  of  sound  by  a 
series  of  recurring  echoes  in  resounding  cavities. 

Of  these  there  are  three  varieties.  First,  the  steam-whistle,  of  which 
the  part  called  the  bell  is  a  resounding  cavity,  the  sound  it  emits  having 
no  relation  to  the  material  of  which  it  is  composed ;  one  of  the  same 
form  and  of  equal  size  of  wood  producing  an  effect  identical  with  that 
from  one  of  metal.  Another  variety  is  the  fog-trumpet,  which  consists 
of  a  trumpet  of  wood  or  metal  actuated  by  a  reed  like  that  of  a  clario- 
net. Tlte  third  variety  is  called  the  siren  trumpet,  which  consists  of  a 
hollow  drum,  into  one  head  of  which  is  inserted  a  pipe  from  a  steam- 
boiler,  while  in  the  other  head  a  number  of  holes  are  pierced,  which  are 
alternately  opened  and  shut  by  a  revolving  plate  having  an  equal  num- 
ber of  holes  through  it.  This  drum  is  placed  at  the  mouth  of  a  large 
trumpet.  The  sound  is  produced  by  the  series  of  impulses  given  to  the 


496  EESEARCHES   IN    SOUND. 

air  by  the  opening  and  shutting  of  the  orifices  and  consequent  rushing  out 
at  intervals  with  explosive  violence  of  the  steam  or  condensed  air.  The 
instrument,  as  originally  invented  by  Cagniard  de  Latour,  of  France, 
was  used  simply  in  experiments  in  physics  to  determine  the  pitch  of 
sound  $  but  Mr.  Brown,  of  New  York,  after  adding  a  trumpet  to  it,  and 
modifying  the  openings  in  the  head  of  the  drum  and  the  revolving  plate, 
offered  it  to  the  Light-House  Board  as  a  fog-signal,  and  as  such  it  has 
been  found  the  most  powerful  ever  employed. 

In  ascertaining  the  penetrating  power  of  different  fog-signals,  I  have 
used  with  entire  success  an  instrument  of  which  the  following  is  a 
description:  A  trumpet  of  ordinary  tinned  iron  of  about  3  feet  in 
length,  and  9  inches  in  diameter  at  the  larger  end,  and  about  1  inch  at 
the  smaller,  is  gradually  bent  so  that  the  axis  of  the  smaller  part  is  at 
right  angles  to  the  axis  of  the  larger  end  5  on  the  smaller  end  is  soldered 
a  cgne,  of  which  the  larger  end  is  about  2  inches  in  diameter.  Across 
the  mouth  of  this  cone  is  stretched  a  piece  of  gold-beater's  skin.  When 
the  instrument  is  used,  the  opening  on  the  larger  end  is  held  before 
the  iustrment  to  be  tested,  the  membrane  being  horizontal,  and  the  mouth 
of  the  trumpet  vertical ;  over  the  membrane  is  strewed  a  small  quantity 
of  fine  sand,  which  is  defended  from  the  agitation  of  the  air  by  a  cylinder 
of  glass,  the  upper  end  of  which  is  closed  by  a  lens.  When  the  instru- 
ment under  examination  is  sounded,  being  sufficiently  near  the  sand,  is 
agitated  ;  it  is  then  moved  further  off,  step  by  step,  until  the  agitation 
just  ceases ;  this  distance,  being  measured,  is  taken  as  the  relative  pene- 
trating power  of  the  sounding  instrument.  The  same  process  is  repeated 
with  another  sounding  instrument,  and  the  distance  at  which  the  sound 
ceases  to  produce  an  effect  on  the  sand  is  taken  as  the  measure  of  the 
penetrating  power  of  this  instrument,  and  so  on.  On  comparing  the  re- 
sults given  by  this  instrument  with  those  obtained  by  the  ear  on  going  out 
a  sufficient  distance,  the  two  are  found  to  agree  precisely  in  their  indica- 
tions. The  great  advantage  in  using  this  contrivance  is  that  the  rela- 
tive penetrating  power  of  two  instruments  may  be  obtained  within  a 
distance  of  a  few  hundred  yards,  while  to  compare  the  relative  power 
of  two  fog-signals  by  the  ear  requires  the  aid  of  a  steamer  and  a  de- 
parture from  the  origin  of  sound  in  some  cases  of  15  or  20  miles. 


PART  III.— INVESTIGATIONS  DURING  1873  AND  1874.* 

OBSERVATIONS  ON  SOUND  AND  FOG-SIGNALS,  IN  AUGUST,  1873. 

• 
Professor  Henry,  chairman,  and  Commander  Walker,  naval  secretary 

of  the  Light-House  Board,  left  Portland  August  12th,  1873,  at  3  o'clock 
p.  M.  in  the  steam-tender  Myrtle,  Captain  Foster,  for  Whjtehead  light- 
station,  at  which  place  abnormal  phenomena  of  sound  had  been  observed. 

*  From  the  Report  of  the  Light- House  Boajxl,,  foj;  1874. 


RESEARCHES    IN    SOUND.  497 

WJiitehead  light-station  is  on  a  small  island  about  a  mile  and  a  half 
from  the  coast  of  Maine,  on  the  western  side  of  the  entrance  to  Penob- 
scot  Bay,  and  in  the  direct  line  of  the  coasting-steamers  and  other  ves- 
sels from  the  westward  bound  into  the  Penobscot  Bay  and  Biver.  The 
light-house  and  fog-signal  are  situated  on  the  southeast  slope  of  the 
island,  the  surface  of  which  consists  almost  entirely  of  rock,  the  middle 
being  at  an  elevation  of  75  feet  above  the  mean  tide-level. 

The  phenomena  which  had  been  observed  at  this  and  other  stations 
along  the  coast  consisted  of  great  variation  of  intensity  of  sound  while 
approaching  and  receding  from  the  station.  As  an  example  of  this  we 
may  state  the  experience  of  the  observers  on  board  the  steamer  City  of 
Bichmond  on  one  occasion,  during  a  thick  fog  in  the  night  in  1872.  The 
vessel  was  approaching  Whitehead  from  the  southwestward,  when,  at  a 
distance  of  about  six  miles  from  the  station,  the  fog-signal,  which  is  a 
10-inch  steam-whistle,  was  distinctly  perceived  and  continued  to  be 
heard  with  increasing  intensity  of  sound  until  within  about  three  miles, 
when  the  sound  suddenly  ceased  to  be  heard,  and  was  not  perceived 
again  until  the  vessel  approached  within  a  quarter  of  a  mile  of  the  sta- 
tion, although  from  conclusive  evidence  furnished  by  the  keeper  it  was 
shown  that  the  signal  had  been  sounding  during  the  whole  time.  The 
wind  during  this  time  was  from  the  south,  or  approximately  in  an  opp.o- 
site  direction  to  the  sound.  Another  fact  connected  with*  this  occur- 
rence was  that  the  keeper  on  the  island  distinctly  heard  the  sound  of 
the  whistle  of  the  steamer,  which  was  commenced  to  be  blown  as  soon 
as  the  whistle  at  the  station  ceased  to  be  heard,  in  order  to  call  the  at- 
tention of  the  keeper  to  what  was  supposed  to  be  a  neglect  of  his  duty 
in  intermitting  the  operations  of  his  signal.  It  should  be  observed  in 
this  case  that  the  sound  from  the  steamer  was  produced  by  a  6-inch 
whistle,  while  that  of  the  station  was  from  an  instrument  of  the  same 
kind  of  10  inches  in  diameter ;  or,  in  other  words,  a  lesser  sound  was 
heard  from  the  steamer,  while  a  sound  of  greater  volume  was  unheard 
in  an  opposite  direction  from  the  station.  It  is  evident  that  this  result 
could  not  be  due  to  any  mottled  condition  or  want  of  acoustic  transpar- 
ency of  the  atmosphere,  since  this  would  absorb  the  sound  equally  in 
both  directions.  The  only  plausible  explanation  of  this  phenomenon 
is  that  which  refers  it  to  the  action  of  the  wind.  In  the  case  of  the 
sound  from  the  steamer,  the  wind  was  favorable  for  its  transmission, 
and  hence  it  is  not  strange  that  its  sound  should  be  heard  on  the  island 
when  the  sound  from  the  other  instrument  could  not  be  be  heard  on  the 
steamer.  To  explain  on  the  same  principle  the  fact  of  the  hearing  of 
the  sound  at  the  distance  of  six  miles,  and  afterward  of  losing  it  at  the 
distance  of  three  miles,  we  have  only  to  suppose  that  in  the  first  in- 
stance the  retarding  effect  of  the  wind  was  small,  and  tj^at  in  the  sec- 
ond it  became  much  greater  on  account  of  a  sudden  increase  in  the  rel- 
ative velocity  of  the  current  in  the  upper  and  lower  portions. 

After  making  a  critical  examination  of  the  island  and  the  position  of 
S.  Mis.  59 32 


498  RESEARCHES   IN   SOUND. 

the  machinery,  and  also  in  regard  to  any  obstacle  which  might  interfere 
with  the  propagation  of  the  sound,  the  keeper  was  directed  to  put  the 
instrument  in  operation  and  to  continue  to  sound  it  for  at  least  two 
hours,  or  until  the  steamer  was  lost  sight  of,  which  direction  was  com- 
plied with.  In  passing  from  the  island,  almost  directly  against  a  light 
wind,  the  intensity  of  the  sound  gradually  diminished  as  a  whole,  with 
the  increase  of  distance,  but  varied  in  loudness  from  blast  to  blast, 
now  louder,  then  again  more  feeble,  until  it  finally  ceased  at  a  distance 
of  about  fifteen  miles,  as  estimated  by  the  intervals  between  the  blasts 
and  the  sight  of  the  steam  as  seen  through  a  spy-glass,  and  also  from 
points  on  the  Coast-Survey  charts. 

The  result  of  this  investigation  clearly  showed  the  power  of  the  appa- 
ratus in  propagating  sound  under  conditions  not  entirely  favorable, 
since  the  wind,  though  light,  was  in  opposition  to  the  sound. 

Cape  Elizabeth  Light- Station,  Maine,  August  29,  1873.— The  fog-signal 
at  this  place  is  on  a  prominent  headland  to  which  the  course  of  all  ves- 
sels is  directed  when  bound  from  the  southward  into  Portland  Harbor. 
It  is  furnished  with  two  light  houses  919  feet  apart  and  143  feet  above 
sea-level.  The  easterly  tower  is  connected  with  the  keeper's  dwelling 
by  a  wooden-covered  way  200  feet  long  and  about  12  feet  high ;  the 
station  is  furnished  with  a  10-inch  steam  fog-whistle,  placed  to  the 
southward  of  the  easterly  tower,  at  a  distance  of  about  625  feet  and 
about  at  right  angles  with  the  covered  way ;  it  therefore  has  a  back- 
ground, including  the  covered  way,  of  about  65  feet  above  the  height  of 
the  whistle,  which  was  found  to  reflect  a  perceptible  echo.  The  whistle 
was  actuated  by  steam  at  55  pounds  pressure,  consuming  from  60  to  65 
pounds  of  anthracite  coal  per  hour.  The  whistle  itself  differs  from  the 
ordinary  locomotive-whistle  by  having  a  projecting  ledge  or  rim  around 
the  lower  part  through  which  the  sheet  of  steam  issues  to  strike  against 
the  lower  edge  of  the  bell.  What  effect  this  projecting  ledge  or  rim  may 
have  is  not  known  to  the  observers.  This  whistle  is  provided,  (for  the 
purpose  of  concentrating  the  sound  in  a  given  direction,)  with  a  hollow 
truncated  pyramid  20  feet  long,  10  feet  square  at  the  large  end,  and  2J 
feet  square  at  the  small  end,  the  axis  of  the  pyramid  being  placed  par- 
allel to  the  horizon,  with  the  whistle  at  the  smaller  end.  In  order  to 
ascertain  the  effect  of  this  appendage  to  the  whistle  the  simplest  plan 
would  have  been  to  have  noted  the  intensity  of  sound  at  various  points 
on  a  circle  of  which  the  whistle  would  have  been  the  center.  This  being 
impracticable  on  account  of  the  intervention  of  the  land,  the  observations 
were  confined  to  points  on  the  three  arcs  of  a  circle  of  about  120°,  of 
which  the  axis  divided  the  space  into  80°  and  40°  and  a  radius  of  one, 
two,  and  three  miles.  The  result  of  these  observations  was  that,  starting 
from  the  axis  of  the  trumpet  on  the  east  side,  the  sound  grew  slightly 
less  loud  until  the  prolongation  of  the  side  of  the  trumpet  was  reached, 
when  it  became  comparatively  faint  and  continued  so  until  the  line  be- 


RESEARCHES    IN    SOUND.  499 

tween  the  whistle  and  observer  was  entirely  unobstructed  by  the  side 
of  the  trumpet,  when  the  sound  was  apparently  as  loud  as  in  the  pro- 
longation of  the  axis  itself.  On  the  west  side,  of  the  axis  of  the  trumpet 
the  sound  in  a  like  manner  diminished  from  the  axis  until  the  prolonga- 
tion of  the  side  of  the  trumpet  was  reached,  when  it  became  feeble 
again,  slightly  increased,  and  then  gradually  diminished  until  the  line 
of  direc  ion  made  an  angle  of  about  80°  with  the  axis  of  the  trumpet, 
when  it  ceased  to  be  heard  at  a  distance  of  about  one  and  a  half  miles. 
It  should  be  observed,  however,  that  at  this  point  the  line  of  sight  of 
the  observers  was  obstructed  by  the  side  of  the  trumpet  and  the  smoke- 
stack of  the  boiler.  The  wind  was  light,  at  south-southwest,  approxi- 
mately in  direct  opposition  to  the  direction  of  the  sound  when  it  ceased 
to  be  heard.  We  are  informed  that  complaints  had  previously  been  made 
by  officers  of  steamers  passing  near  this  point  that  the  sound  was  here 
inaudible  previous  to  the  introduction  of  this  trumpet  ;  it  would  there- 
fore follow  that  it  is  of  no  use  in  increasing  the  effect  on  the  western 
side  of  the  axis  and  is  of  injury  to  the  sound  on  the  lines  of  prolonga- 
tion of  its  sides.  If  the  sound  ceased  to  be  heard  at  the  point  mentioned, 
when  the  trumpet  is  removed  the  only  apparent  cause  of  the  phenome- 
non will  be  the  prevailing  direction  of  the  wind,  which,  coming  from  the 
southwest,  will  be  in  opposition  to  the  sound  of  the  whistle ;  but  in  the 
case  of  the  present  investigation  the  force  of  the  wind  was  so  small  that 
it  scarcely  appeared  adequate  to  produce  the  effect,  and  this  question, 
therefore,  must  be  left  for  further  investigation.  It  may  be  important 
to  state  that  in  the  case  where  the  sound  ceased  to  be  heard  it  was  re- 
gained by  sailing  directly  toward  the  station  about  one  mile,  or  at  half 
a  mile  from  the  station.  After  making  the  foregoing  observations  as  to 
the  intensity  of  sound  in  different  directions  from  the  station,  the  obser- 
vations were  closed  by  sailing  directly  along  the  axis  of  the  trumpet 
until  the  sound,  which  gradually  grew  fainter  as  the  distance  increased, 
finally  ceased  to  be  heard  at  a  distance  of  about  nine  miles.  In  com- 
paring this  last  result  with  an  instrument  of  about  the  same  power  at 
Whitehead,  which  gave  a  perceptible  sound  at  a  distance  of  fifteen 
miles,  the  only  apparently  variable  circumstance  was  the  velocity  of  the 
wind,  in  both  cases  adverse  to  the  direction  of  the  sound  ;  but  in  that 
of  Cape  Elizabeth  it  was  of  considerable  more  intensity. 

During  the  foregoing  experiments,  when  the  vessel  was  about  a  mile 
from  the  station,  steaming  directly  outward,  in  the  prolongation  of  the 
axis  of  the  instrument,  there  was  heard  after  each  sound  of  the  whistle 
a  distinct  echo  from  the  broad,  unobstructed  ocean,  which  was  attributed 
at  the  time,  as  in  other  cases,  to  reflections  from  the  crests  and  hollows 
of  the  waves,  a  similar  phenomenon  having  since  been  referred  to  a  re- 
flection from  air  of  a  different  density.  This  observation  becomes  im- 
portant in  regard  to  the  solution  of  the  question  as  to  the  abnormal 
phenomena  of  sound. 


500  KESEARCHES   IN   SOUND. 

Cape  Ann  Light- Station,  Massachusetts,  August  31, 1873. — This  is  one  of 
the  most  important  stations  on  the  New  England  coast.  It  is  furnished 
with  two  first-order  lights,  and  a  12-inch  steam- whistle,  actuated  by  60 
pounds  pressure  of  steam.  The  present  is  the  fourth  engine  which  has 
been  erected  at  this  station,  in  consequence  of  the  complaints  either  as 
to  the  inefficiency  of  the  sound  or  its  failure  to  be  heard  in  certain  direc- 
tions. It  was  at  first  proposed  to  sail  entirely  around  the  island  in  order 
to  test  the  intensity  of  the  sound  in  different  directions,  but  this  was 
i  found  impracticable  on  account  of  want  of  depth  of  water  on  the  inland 
Wde ;  the  observations  were  therefore  confined  to  the  direction  in  which 
complaints  had  been  made  as  to  the  deficiency  of  the  signal,  namely,  in 
a  southerly  direction.  The  result  of  these  observations,  the  points  of 
which  included  an  arc  of  120°,  was  that  the  sound  was  heard  with  equal 
intensity  except  when  the  direction  of  the  station  was  to  the  northward 
and  eastward  of  tke  observers ;  then,  in  one  instance,  the  sound  became 
very  indistinct,  and  in  another  was  entirely  lost,  both  at  a  distance  of 
about  two  miles.  In  these  cases  the  line  of  sight  between  the  observers 
and  the  signal  was  interrupted,  in  the  first  by  a  small  building,  the  gable- 
end  of  which  was  within  10  feet  of  the  whistle,  and  in  the  second  by  the 
south  light-tower,  which  is  within  30  feet  of  the  whistle.  In  this  series 
of  experiments,  as  with  the  last,  the  wind  was  against  the  sound ;  the 
effect  was  noted  by  passing  over  the  arc  several  times  at  different  dis- 
tances. The  wind  was  from  the  southward  and  westward  and  very 
light,  and  the  sound  was  finally  lost  at  about  six  miles,  and  in  the  direc- 
tion of  the  obstructions. 

Boston  Light- Station,  August  31,  1873. — The  light-house  is  situated  on 
a  low,  rocky  island,  on  the  north  side  of  the  main  outer  entrance  to  Bos- 
ton Harbor,  nine  miles  from  the  city.  It  is  furnished  with  three  caloric 
engines^  two  of  the  second  class  and  one  of  the  first.  The  two  second- 
class  engines  are  so  arranged  as  to  act  separately  or  together,  and  in  the 
latter  arrangement  serve  to  duplicate  the  larger  engine.  At  the  time 
the  observations  were  made,  the  larger  engine  was  about  being  repaired, 
and  one  of  the  smaller  engines  with  the  double  air-reservoir  was  used. 
The  larger  engine  is  used  with  12  pounds  pressure  of  air,  which  falls  to 
8  pounds  in  producing  the  sound.  The  smaller  engine,  with  the  double 
reservoir,  is  started  with  i)  pounds  pressure,  which  falls  to  8  pounds. 
This  difference  in  the  pressure  of  air  in  the  two  engines  is  caused  by  the 
larger  ratio  of  the  reservoir  to  the  size  of  the  reed.  With  a  greater 
pressure  than  12  pounds  to  the  square  inch  in  the  larger  engine  and  9 
pounds  in  the  smaller  no  sound  is  produced ;  the  reed  is  unable  to  act 
against  the  pressure,  and,  consequently,  the  orifice  remains  closed.  The 
trumpet  of  the  larger  of  the  engines  is  reported  to  have  been  heard 
eighteen  miles  at  sea,  which,  in  consideration  of  the  results  obtained  at 
Whitehead,  we  thought  very  probable.  The  time  required,  from  start- 
ing fires,  to  get  a  good  working-pressure,  is  about  half  an  hour.  The 
amount  of  coal  consumed  per  hour  is  17  pounds. 


RESEARCHES   IN   SOUND.  501 

There  is  moreover  at  this  station  a  bell,  operated  by  a  Stevens  clock, 
not  at  present  used.  It  is  placed  on  a  high,  wooden  frame-structure,  on 
which  one  of  the  ancient  bell-striking  machines  was  originally  erected. 
The  most  proper  position  for  the  fog- signal  is  on  the  ground  occupied  by 
this  bell-tower,  but  as  this  was  not  removed  at  the  time  of  the  erection 
of  the  trumpets,  they  were  placed  in  such  positions  as  to  have  the  line 
of  sound  interrupted  to  the  northeastward  by  the  bell  and  light  towers. 
It  was  therefore  thought  probable  that  this  was  the  cause  of  the  de- 
ficiency of  sound  in  this  direction.  To  test  this  the  vessel  was  caused 
to  traverse  the  arcs  of  several  concentric  circles,  in  the  portion  of  the 
horizon  where  the  sound  was  most  required  as  a  signal.  The  first  arc 
traversed  was  about  one  and  one-half  miles  from  the  signal.  The  vessel 
on  this  crossed  the  axis  where  the  sound  was  quite  loud,  and  proceeded 
northward  until  the  sight  of  the  trumpet  was  obscured  by  the  before- 
mentioned  towers,  when  the  sound  became  almost  inaudible.  The  ves- 
sel next  returned  across  the  axis,  on  a  circle  of  about  three  miles  radius, 
with  similar  results ;  but  after  crossing  the  axis  the  sound  on  the  south- 
ern side  continued  to  be  but  little  diminished  in  intensity  along  an  arc  of 
two  and  a  half  miles,  or  as  far  as  the  land  would  allow  the  vessel  to  go. 
The  vessel  was  next  put  upon  an  arc,  of  which  the  radius  was  one  and 
a  half  miles,  and  on  the  south  side  of  the  axis,  and  sailed  to  the  north- 
ward until  the  axis  was  reached,  it  was  then  turned  and  ran  for  the  en- 
trance of  the  harbor,  hugging  the  southern  shore,  keeping  as  far  from  the 
signal  as  possible.  Throughout  this  passage  the  sound  was  clear  and 
loud,  showing  very  little,  if  any,  diminution  of  power  as  the  several  posi- 
tions deviated  more  and  more  from  the  direction  of  the  axis,  until  the 
vessel  was  at  right  angles  with  the  axis,  the  land  not  permitting  any 
greater  distance.  The  vessel  approached  to  within  three-quarters  of  a 
mile  of  the  signal  and  then  continued  still  farther  around,  until  nearly  in 
the  rear  of  it,  the  sound  still  continuing  clear  and  loud.  The  vessel  next 
proceeded  up  the  harbor,  nearly  in  the  line  of  the  axis  of  the  trumpet 
prolonged  in  the  rear,  still  continuing  to  hear  the  signal  distinctly  until 
the  keeper,  losing  sight  of  the  vessel,  stopped  sounding  the  instrument. 
These  observations  were  made  under  very  favorable  circumstances,  it 
being  nearly  calm.  What  wind  did  exist  was  about  equally  favorable 
to  points  on  either  side  of  the  axis.  The  inference  from  these  observa- 
tions is,  first,  that  small  objects  placed  near  the  source  of  sound  tend  to 
diminish  its  intensity  in  the  direction  of  its  interruption,  and  should, 
therefore,  if  possible,  be  removed,  or  the  instrument  so  placed  as  to  ob- 
viate such  obstructions ;  and,  second,  that,  even  with  the  trumpet,  the 
sounjl  so  diverges  from  the  axis  as  to  be  efficient  even  in  the  rear  of  the 
instrument. 

OBSERVATIONS  ON  FOG-SIGNALS,  AUGUST  25,  1874. 

The  first  of  these  was  on  board  the  steamer  Putnam,  at  Little  Gull 
Island,  with  Admiral  Trenchard,  inspector  of  lights  of  the  third  dis- 


502  RESEARCHES    IN    SOUND. 

trict,  accompanied  by  Governor  Ingersoll,  of  Connecticut,  and  Captain 
Upshur,  U.  S.  K 

At  this  place  are  two  sirens,  the  one  to  replace  the  other  in  case  of 
an  accident.  One  of  the  sirens  was  sounded  with  the  pressure  of  50 
pounds  per  square  inch.  The  wind  was  across  the  axis  of  the  trumpet, 
and  almost  precisely  at  right  angles  to  it. 

The  steamer  was  headed  against  the  wind,  on  a  line  at  right  angles 
to  the  axis  of  the  trumpet.  The  sound  in  this  case  also  travelled  against 
the  wind,  which  was  at  an  estimated  velocity  of  from  4  to  5  miles  per 
hour.  The  distance  travelled  before  the  sound  became  inaudible  was 
estimated,  by  the  speed  of  the  steamer,  at  3J  miles. 

The  steamer  was  next  headed  in  an  opposite  direction  and  returned 
along  its  previous  path,  across  the  mouth  of  the  trumpet  of  the  siren, 
the  sound  gradually  increasing  in  strength  without  any  marked  irregu- 
larity, until  the  siren  was  reached,  and  on  leaving  this,  the  course  re- 
maining the  same,  the  sound  gradually  diminished  in  intensity,  but  with 
less  rapidity  than  before,  until  it  was  finally  lost  at  a  distance  of  7£ 
miles.  In  the  latter  instance  the  movement  of  the  sound  was  with  the 
wind.  The  result  of  these  observations  was  conformable  to  that  gene- 
rally obtained  from  previous  observations,  namely,  that  the  sound  is  sel- 
dom or  never  heard  at  the  same  distance  in  different  directions,  and,  more- 
over, that  it  is  generally  heard  farther  with  the  wind  than  against  it. 

The  observations  of  this  day  also  illustrate  the  spread  of  the  sound- 
wave on  either  side  of  the  axis  of  the  trumpet,  a  fact  which  has  fre- 
quently been  observed  in  other  investigations.  It  may  be  well  to  men- 
tion that  the  siren  trumpet  at  this  locality  is  directed  horizontally  with 
its  prolonged  axis  passing  over,  immediately  in  front  of  the  mouth  of 
the  trumpet,  a  space  of  very. rough  ground,  the  surface  of  which  is 
principally  composed  of  bowlders,  one  of  which,  of  very  large  size,  is 
directly  in  front  of  the  trumpet,  and  the  idea  occurred  to  me  that  this 
rough  surface  might  produce  some  effect  on  the  transmission  of  sound 
to  a  distance.  I  observed  by  strewing  sand  upon  a  paper  that  the 
former  was  violently  agitated  when  held  near  the  surface  of  the  large 
bowlder  just  mentioned,  during  the  blast  of  the  siren  trumpet. 

At  this  station,  during  the  visit  of  Sir  Frederick  Arrow,  the  sound 
was  lost  in  the  direction  of  the  axis  of  the  trumpet  at  a  distance  of  two 
miles,  and  then  again  regained  with  distinctness  at  the  light- vessel,  a 
distance  of  four  and  one-half  miles ;  this  was  what  we  have  denominated 
as  an  abnormal  phenomenon,  which  we  think  was  due  to  a  slight  varia- 
tion in  the  velocity  of  the  lower  or  upper  part  of  the  current  of  air,  but, 
unfortunately,  the  demand  for  the  use  of  the  vessel  as  a  light-house 
tender  prevented  the  attempt  to  ascertain  whether  the  same  phenomenon 
would  be  observed  a  second  time  and  to  further  investigate  its  cause. 

The  second  investigations  this  season  were  September  1,  1874,  with 
General  Barnard,  of  the  Light- Ho  use  Board,  and  General  Woodruff, 


RESEARCHES   IN    SOUND.  503 

engineer  of  the  third  district.  We  proceeded  on  this  occasion  in  the 
steamer  Mistletoe  to  Block  Island,  one  of  the  outer  stations  of  the 
Light-House  Board,  fully  exposed,  without  intervention  of  land,  to  the 
waves  and  storms  of  the  ocean. 

On  the  southerly  side  of  this  island  a  light-house  is  about  being  erected, 
and  a  siren  station  at  this  locality  had  been  established  and  was  in  full 
operation. 

There  are  here  two  sirens  attached  to  one  boiler,  one  to  be  used  in  case 
of  an  accident  to  the  other.  For  the  sake  of  experiment  they  are  of 
slightly  different  qualities,  one  with  a  larger  trumpet  with  a  revolving 
disk  of  the  old  pattern,  giving  a  lower  tone ;  the  other  a  smaller  trumpet, 
having  a  revolving  disk  with  openings  allowing  a  much  more  sudden 
full  blast  of  steam,  and  revolving  with  greater  velocity  so  as  to  give  a 
higher  pitch.  The  latter  is  far  the  superior  instrument,  as  was  evident 
to  us  by  the  sound  which  it  produced,  and  as  had  been  established  by 
the  use  of  the  artificial  ear  in  the  manufactory  of  Mr.  Brown.  The 
effect  on  the  unguarded  ear  was  scarcely  endurable,  and  the  very  earth 
around  appeared  to  tremble  during  the  blast.  The  keeper  (an  intelli- 
gent man  who  has  been  promoted  from  the  station  of  assistant  keeper 
at  Beaver  Tail  light  to  this  station)  informed  us  that  a  fleet  of  fishing- 
vessels  coming  in  distinctly  heard  it  at  a  distance  estimated  by  their 
rate  of  sailing  at  scarcely  less  than  thirty  miles ;  this  was  on  two  sepa- 
rate occasions.  The  keeper  had  been  directed  to  note  and  record  the 
date  at  which  he  heard  the  sound  from  other  signals  ;  he  reported  that 
he  had  frequently  heard  the  fog-signal  at  Point  Judith,  a  distance  of 
seventeen  miles,  and  that  the  observer  at  the  latter  place  frequently 
heard  his  signal  ;  but  on  comparing  records  the  two  sounds  had  not 
been  heard  simultaneously  by  the  two  keepers ;  when  it  was  heard  from 
one  station  it  was  not  heard  from  the  other,  illustrating  again  the  gen- 
eral rule  that  sound  is  not  transmitted  simultaneously  with  equal  inten- 
sity in  opposite  directions. 

This  occasion  also  furnished  very  favorable  conditions  for  observing 
the  remarkable  phenomenon  of  the  ocean-echo.  At  the  cessation  of 
each  blast  of  the  trumpet,  after  a  slight  interval,  a  distinct  and  pro- 
longed echo  was  returned  from  the  unobstructed  ocean.  It  is  important 
to  observe,  in  regard  to  this  phenomenon,  that  the  siren  is  placed  near 
the  edge  of  a  perpendicular  cliff,  at  an  elevation  of  from  75  to  100  feet 
above  the  ocean,  and,  furthermore,  that  the  direction  of  the  wind  formed 
an  angle  of  about  35°  with  the  axis  of  the  trumpet.  Now,  the  loudness 
of  this  echo  was  not  the  greatest  at  the  siren-house,  but  increased  in 
intensity  until  a  point  was  reached  several  hundred  yards  from  the 
trumpet,  approximately  more  in  accordance  with  a  reflection  from  the 
waves.  The  wind  was  blowing  from  the  shore  with  the  direction  of  the 
sound  as  it  went  off  from  the  trumpet,  and  nearly  against  it  on  the  re- 
turn of  the  echo.  I  have  attributed  this  phenomenon,  which  was  first 
observed  in  18GG  at  East  Quoddy  Head  on  the  coast  of  Maine,  and  since 


504.  RESEARCHES   IN    SOUND. 

at  various  stations,  at  which  the  trumpet  or  siren  has  been  used,  to  the 
reflection  of  the  sound  from  the  crests  and  slopes  of  the  waves,  and  the 
observation  we  have  mentioned  would  appear  to  favor  this  hypothesis. 
In  connection  with  this  explanation,  I  may  mention  that  my  attention 
has  been  called  by  General  Meigs,  of  the  United  States  Army,  to  an 
echo  from  the  palings  of  a  fence,  and  also  from  a  series  of  indentations 
across  the  under  side  of  the  arch  of  one  of  the  aqueduct  bridges  of  the 
Washington  water- works.  The  fact  that  the  sound  was  much  louder  at 
a  point  considerably  distant  from  the  trumpet  was  noted  by  one  of  the 
party  entirely  unacquainted  with  the  hypothesis. 

The  keeper  at  this  station  confirmed  without  a  leading  question  the 
statement  of  Captain  Keeney,  that  it  frequently  happens  that  a  feeble 
sound  of  a  distant  object,  as  the  roar  of  the  surf,  can  be  heard  against 
the  direction  of  the  wind,  and  that  in  this  case  it  always  betokens  a 
change  in  the  weather,  and  is,  in  fact,  used  generally  by  the  fishermen 
as  a  prognostic  of  a  change  in  the  direction  of  the  wind,  which  will,  in 
the  course  of  a  few  hours,  invariably  spring  up  from  an  opposite  quar- 
ter. In  such  case,  it  is  highly  probable,  as  has  been  stated,  that  a  change 
has  already  taken  place  in  the  direction  of  the  upper  strata  of  the  air, 
although,  from  theoretical  considerations,  we  might  infer  that  the  same 
result  would  be  produced  if  the  wind  were  stationary  above  and  moving 
with  a  considerable  velocity  in  a  direction  opposite  to  the  sound  at  the 
surface  of  the  earth,  the  velocity  gradually  diminishing  as  we  ascend, 
for  in  this  case,  also,  the  inclination  of  the  sound  waves  would  be  down- 
ward. 

The  third  series  of  investigations,  September  23,  24, 1874,  was  made 
in  company  with  Captain  John  Davis  and  Major  Hains,  both  of  the 
Light-House  Board,  and  General  Woodruif,  engineer  of  the  third  dis- 
trict, and  Mr.  Brown,  patentee  of  the  siren.  For  the  purpose  three 
light-house  tenders  were  employed,  viz:  Mistletoe,  Captain  Keeney j 
Putnam,  Captain  Field ;  Cactus,  Captain  Latham. 

The  place  of  operation  chosen  for  the  first  day's  series  was  about  1J 
miles  from  the  northern  point  of  Sandy  Hook. 

From  the  experience  gained  by  the  accumulated  observations  which 
had  been  made,  it  was  concluded  that  the  phenomena  of  sound  in  re- 
gard to  perturbing  influences  could  not  be  properly  studied  without 
simultaneously  observing  the  transmission  of  sound  in  opposite  direc- 
tions. It  was  therefore  concluded  to  employ  at  least  two  steamers  in 
making  the  investigations. 

In  regard  to  this  point  the  commission  was  fortunate  in  being  able  to 
command  the  use,  for  a  limited  period,  of  the  three  tenders  mentioned 
above,  which  happened  to  be  at  the  time  assembled  at  the  light-house 
depot,  Staten  Island,  and  could  be  spared  from  their  ordinary  opera- 
tions for  a  few  days  without  detriment  to  the  service.  It  was  also  for- 
tunate in  selecting  for  the  scene  of  the  investigations  an  unobstructed 


RESEARCHES   IN    SOUND.  505 

position  in  the  lower  bay  of  ]Sew  York,  and  perhaps  still  more  fortunate 
in  the  season  of  the  year  when,  on  account  of  the  heat  of  the  snu,  a  land 
and  sea  breeze,  which  changed  its  direction  at  a  particular  hour  of  the 
day,  enabled  results  to  be  obtained  bearing  especially  on  the  phenomena 
to  be  investigated. 

Attention  was  first  given  to  the  character  of  the  several  steam-whistles 
which  were  intended  to  be  used  as  the  sources  of  the  sound  during  the 
series  of  investigations. 

These  whistles,  which  were  sounded  during  the  whole  of  the  observa- 
tions with  20  pounds  of  steam  on  each  boiler,  gave  at  first  discordant 
sounds,  and  were  found  by  their  effect  upon  an  artificial  ear  to  be  con- 
siderably different  in  penetrating  power;  they  were  then  adjusted  by 
increasing  or  diminishing  the  space  between  the  bell  and  the  lower  cylin- 
der by  turning  a  screw  on  the  axis  of  the  bell  intended  for  that  purpose, 
until  they  produced  the  same  effect  upon  the  sand  in  the  membrane  of 
the  artificial  ear;  but  in  order  to  further  be  insured  of  the  equality  of 
the  penetrating  power  of  the  several  whistles,  the  three  steamers  abreast, 
forming  as  it  were  a  platoon,  were  directed  to  proceed  against  the  wind, 
sounding  all  the  time  in  regular  succession — the  Cactus  first,  then,  after 
an  interval  of  a  few  seconds,  the  Mistletoe,  and  then  the  Putnam — until 
the  stationary  observers  lost  the  sound  of  each.  They  became  inaudible 
all  very  nearly  at  the  same  moment.  The  sound  of  the  Putnam  was 
thought  to  be  slightly  less  distinct ;  it  was  therefore  chosen  as  a  station- 
ary vessel,  from  which  the  observations  of  the  sound  of  the  other  two 
were  to  be  made. 

The  Putnam  being  anchored  at  the  point  before  mentioned,  arrange- 
ments were  made  for  sending  off  the  other  two  vessels  in  opposite  direc- 
tions, one  with  and  the  other  against  the  wind,  with  instructions  to  return 
when  the  sound  became  inaudible  to  those  on  the  stationary  vessel,  this 
to  be  indicated  by  a  flag-signal.  It  should  be  mentioned  that  the  velocity 
of  the  wind  was  measured  from  time  to  time  during  the  subsequent  ex- 
periments with  one  of  Eobinson's  hemispherical  cup  anemometers,  made 
by  Casella,  of  London.  The  velocity  of  the  wind  first  observed  by  this 
instrument,  just  before  the  starting  of  the  vessels,  was  6  miles  per  hour, 
the  instrument  being  freely  exposed  on  the  paddle-boxes  of  the  steamer. 
A  sensitive  aneroid  barometer  marked  30.395  in.  and  continued  to  rise 
gradually  during  the  day  to  30.43  in.  the  temperature  was  71°  F. 

The  vessels  left  at  11:18  A.  M.  the  wind  being  from  the  west,  Captain 
Davis  taking  charge  of  the  sounding  of  the  whistle  on  the  Cactus,  which 
proceeded  east  with  the  wind,  the  sound  coming  to  the  ear  of  the  observer 
Against  the  wind;  while  the  sounding  on  the  Mistletoe  was  in  charge  of 
General  Woodruff,  and,  as  the  vessel  steamed  against  the  wind,  the  sound 
came  to  the  observers  on  the  stationary  vessel  with  the  wind ;  the  other 
members  of  the  party  remained  on  the  Putnam,  at  anchor  at  the  point 
before  mentioned,  off  the  Hook,  Major  Hams  having  charge  of  the  sig- 
nals. The  sound  of  the  first  of  the  vessels  was  heard  faintly  at  14  min- 


506  EESEAECHES   IX   SOUND. 

utes  after  leaving,  but  not  heard  at  16  minutes;  we  may  therefore  assume 
that  it  became  inaudible  at  15  minutes.  And  within  a  minute  of  the 
same  time,  by  a  mistake  of  the  signal,  the  other  ceased  to  advance,  and 
commenced  to  come  back ;  the  sound  from  it,  however,  was  very  distinct, 
while  at  the  same  moment  the  sound  from  the  other  was  inaudible.  On 
account  of  the  mistake  mentioned,  the  relative  distance  at  which  the 
sounds  from  the  two  vessels  might  have  become  inaudible  cannot  be 
accurately  given;  but  the  fact  observed,  that  the  sound  which  came 
with  the  wind  was  much  more  audible  than  the  other,  is  in  conformity 
with  the  generally  observed  fact  that  sound  is  heard  farther  with  the 
wind  than  against  it.  In  the  mean  time  the  velocity  of  the  wind  iiad 
sunk  to  1 J  miles  per  hour. 

Next,  the  vessels,  leaving  at  11:55  A.  M.  changed  positions ;  the  Cac- 
tus, under  Captain  Davis,  steamed  west,  directly  in  the  direction  from 
which  the  wind  caine,  while  the  Mistletoe,  under  General  Woodruff, 
steamed  east,  directly  before  the  wind.  The  result  of  this  trial  was  well 
marked  in  all  respects ;  the  sound  of  the  Mistletoe  was  lost  in  9  minutes, 
which,  from  the  speed  of  the  steamer,  was  estimated  at  about  1£  miles, 
while  the  sound  of  the  Cactus  was  heard  distinctly  for  30  minutes,  or 
at  an  estimated  distance  of  5  miles.  The  wind  at  the  middle  of  this 
trial  had  sunk  to  0.42  mile  per  hour,  or  nearly  to  a  calm.  The  result  of 
this  trial  was  somewhat  abnormal,  for  though  the  wind  had  sunk  nearly 
to  a  calm,  the  sound  was  still  heard  three  times  as  far  in  the  direction  of 
the  slight  wind  as  against  it. 

After  a  lapse  of  an  hour  and  a  half  a  third  trial  was  made  ,•  in  the 
mean  time  the  wind  had  changed  within  two  points  of  an  exactly 
opposite  direction,  blowing,  from  the  indications  of  the  anemometer,  at 
the  rate  of  ten  and  one-half  miles  per  hour. 

The  Cactus  again  steamed  in  the  eye  of  the  wind,  which  was  now 
however  from  nearly  an  opposite  point  of  the  compass,  while  the  other 
vessel  steamed  in  an  opposite  direction.  The  sound  of  the  Cactus  was 
lost  at  the  end  of  twenty-seven  minutes,  with  the  wind,  or  at  a  distance 
of  four  and  a  half  miles. 

The  sound  of  the  Mistletoe  was  lost  at  the  end  of  thirty  minutes,  or 
at  a  distance  of  five  miles,  moving  against  a  brisk  wind  then  blowing. 

This  result  was  entirely  unexpected  and  much  surprised  every  member 
of  the  party,  since  it  was  confidently  expected  that  an  increase  in  the 
intensity  of  the  wind  of  more  than  ten  miles  per  hour,  and  a  change  to 
the  opposite  direction,  would  materially  affect  the  audibility  of  the 
sound,  and  give  a  large  result  in  favor  of  the  sound,  which  moved  in 
the  same  direction  with  the  wind,  but  this  was  not  the  case.  In  the 
course  of  all  the  observations  in  several  years  in  which  investigations 
have  been  carried  on  under  the  direction  of  the  chairman  of  the  board, 
this  is  the  only  instance  in  which  he  had  heard  a  sound  at  a  greater  dis- 
tance against  the  wind  than  with  it,  although,  as  before  stated,  a  mm- 


RESEARCHES    IN    SOUND.  507 

ber  of  eases  have  been  reported  by  other  observers  in  which,  under 
peculiar  conditions  of  the  weather,  this  phenomenon  has  been  observed. 

To  briefly  recapitulate  the  results,  we  have  in  this  case  three  instances, 
in  succession,  in  which  a  sound  was  heard  farther  from  the  west  than 
from  the  east,  although  in  the  mean  time  the  wind  had  changed  to  nearly 
an  opposite  direction.  Had  these  results  been  deduced  from  the  first 
observations  made  on  the  influence  of  wind  on  sound,  or,  in  other 
words,  without  previous  experience,  the  conclusion  would  have  been 
definitely  reached  that  something  else  than  wind  affected  the  convey- 
ance of  sound,  and  this  conclusion  would  have  been  correct,  if  the  sug- 
gestion had  been  confined  to  the  wind  at  the  surface ;  but  from  previous 
observations  and  theoretical  conclusions,  the  observed  phenomena  aro 
readily  accounted  for  by  supposing  that  during  the  whole  time  of  ob- 
servation the  wind  was  blowing  from  the  west  in  the  higher  part  of  the 
aerial  current,  and  that  the  calm  and  opposing  wind  observed  were  con- 
fined to  the  region  near  the  surface.  To  test  this  hypothesis,  Major 
Hains  constructed  a  balloon  of  tissue-paper,  which,  after  being  com- 
pleted, was  unfortunately  burned  in  the  attempt  to  inflate  it  with  heated 
air. 

The  remainder  of  this  day  was  devoted  to  observations  on  the  sound 
of  the  siren  at  the  light-house  at  Sandy  Hook.  For  this  purpose  the 
Cactus,  under  Captain  Davis,  was  directed  to  steam  in  the  eye  of  the 
wind,  while  the  Mistletoe,  under  General  Woodruff,  steamed  before  the 
wind,  and  the  Putnam  steamed  at  right  angles  to  the  wind.  Unfor- 
tunately, on  account  of  the  diminution  of  light  at  the  closing  in  of  the 
day,  nothing  could  be  observed.  The  only  result  obtained  was  that  one 
of  the  duplicate  sirens  was  heard  more  distinctly  than  the  other,  namely, 
the  one  with  the  higher  note. 

Experiments  September  24,  1874. — The  place  chosen  for  the  observa- 
tions of  this  day  was  still  farther  out  in  the  ocean,  at  the  Sandy  Hook 
light- vessel,  6  miles  from  the  nearest  point  of  land.  The  pressure  of  the 
atmosphere  was  a  little  greater  than  the  day  before,  being  30.52 ;  the 
temperature  about  the  same,  72°  Fahr.  wind  light,  from  a  westerly 
direction,  as  on  the  previous  day,  with  a  force,  as  indicated  by  the 
anemometer,  of  1.2  miles  per  hour.  Having  been  provided  with  a  num- 
ber of  India-rubber  toy  balloons,  the  two  vessels  were  sent  off  in  opposite 
directions — the  Mistletoe  toward  the  west,  against  the  wind,  the  Cactus 
toward  the  east,  with  the  wind,  leaving  at  10:40  A.  M.  A  change  was 
also  made  in  observing  the  sound.  In  these  observations  the  sound  was 
noted  at  each  vessel  from  the  other,  the  speed  of  the  steamers  being  the 
same ;  the  distance  between  them  when  the  Mistletoe  lost  the  sound  of 
the  Cactus  was  two  miles,  while  the  Cactus  continued  to  hear  the  Mistle- 
toe's sound  coming  with  the  wind  until  they  were  four  miles  apart.  Simul- 
taneously with  this  observation  a  balloon  was  let  off  from  the  Putnam 
at  the  light-vessel,  which,  in  its  ascent,  moved  continuously  obliquely 


508  RESEARCHES    IN   SOUND 

upward  in  a  line  slightly  curving  toward  the  horizon,  in  the  direction  of 
the  wind  at  the  surface,  as  far  as  it  could  be  followed  with  the  eye,  in- 
dicating a  wind  in  the  same  direction  in  the  several  strata  through  which 
it  passed,  but  of  a  greater  velocity  in  the  upper  strata. 

The  vessels  now  changed  places,  the  Cactus  steaming  west,  the  Mis- 
tletoe east,  the  wind  having  entirely  ceased  at  the  surface  of  the  earth. 
In  this  case  the  Cactus  lost  the  sound  of  the  Mistletoe  when  the  vessels 
were  two  miles  apart,  while  the  Mistletoe  continued  to  hear  the  sound 
of  the  Cactus  until  they  were  three  miles  apart.  A  balloon  let  off  as- 
cended vertically  until  it  attained  an  elevation  of  about  one  thousand 
feet,  when,  turning  east,  it  followed  the  direction  of  the  previous  one. 
The  sound  in  this  case  from  the  east  was  heard  three  miles,  while  that 
from  the  west  was  heard  two  miles,  while  in  the  preceding  observations 
the  distances  were  as  2  to  1 ;  the  only  changing  element,  as  far  as  could 
be  observed,  was  that  of  the  wind  at  the  surface,  which  became  less. 

Third  trial,  12:45  P.  M. — The  wind  previous  to  this  trial  had  changed 
its  direction  10  points  or  about  112£°  round  through  the  south,  and  as 
indicated  by  the  anemometer  at  a  velocity  of  4.8  miles  per  hour.  In 
this  case  the  Cactus,  going  against  the  wind,  lost  the  Mistletoe's  sound 
coming  to  her  against  the  wind  when  the  vessels  were  1  mile  apart, 
while  the  Mistletoe  heard  the  Cactus,  the  sound  coming  to  her  with  the 
wind  when  the  vessels  were  1J-  miles  apart.  The  several  balloons  set 
off  at  this  time  were  carried  by  the  surface  wind  westwardly  until  nearly 
lost  to  sight,  when  they  were  observed  to  turn  east,  following  the  direc- 
tion of  the  wind  observed  in  the  earlier  observations.  The  results  of 
the  whole  series  of  observations  are  extremely  interesting.  In  all  the 
experiments  the  difference  in  the  audibility  of  the  sound  in  different  di- 
rections was  very  marked,  and  indeed  it  rarely  happens  that  the  sound 
is  equal  in  two  directions,  although  from  the  hypothesis  adopted  this 
may  be  possible,  since,  according  to  this  hypothesis,  both  the  upper  and 
lower  currents  have  an  influence  upon  the  audibility  of  sound  in  certain 
directions.  From  the  first  trial,  the  motion  of  the  air  being  in  the  same 
direction  both  below  and  above,  but  probably  more  rapid  above  than 
below  on  account  of  resistance,  the  upper  part  of  the  sound-wave  would 
move  more  rapidly  than  the  lower,  and  the  wave  would  be  deflected 
downward,  and  therefore  the  sound,  as  usual,  heard  farther  with  the  wind 
than  against  it.  In  the  third  experiment  of  the  same  day,  in  which  the 
wind  changed  to  an  almost  opposite  direction,  if  the  wind  remained  the 
same  above,  as  we  have  reason  to  suppose  it  did  from  the  observations 
on  the  balloons  on  the  second  day,  the  sound  should  be  heard  still 
farther  in  the  same  direction  or  against  the  wind  at  the  surface,  since, 
in  this  case,  the  sound-wave  being  more  retarded  near  the  surface  would 
be  tipped  over  more  above  and  the  sound  thus  be  thrown  down. 

The  observations  of  the  second  day  are  also  in  conformity  with  the 
same  hypothesis,  the  change  in  the  wind  being  probably  due  to  the' 
heating  of  the  land,  as  the  day  advanced,  beyond  the  temperature  of 


RESEARCHES   IN   SOUND.  509 

9 

the  water,  and  thus  producing1  a  current  from  the  latter  to  the  former, 
while  the  wind  observed  in  the  morning  from  the  west  was  the  land- 
wind  due  to  the  cooling  of  the  latter. 

In  the  morning  the  wind  was  blowing  from  the  wesk  both  in  the 
higher  strata  and  at  the  surface  of  the  earth,  and  in  this  condition  the 
sound  was  heard  farther  with  the  wind  than  against  it. 

The  wind  at  the  surface  about  midday  gradually  ceased,  and  shortly 
afterward  sprang  up  from  an  east  direction  5  in  this  condition  the  sound, 
with  the  wind  at  the  surface,  was  heard  at  a  greater  distance.  This  is 
also  in  strict  conformity  with  the  theory  of  a  change  in  the  form  of  the 
sound- wave,  as  in  the  latter  case  the  lower  portion  would  be  retarded, 
while  the  upper  portion  of  the  wave  would  be  carried  forward  with  the 
same  velocity,  and  hence  the  sound  would  be  thrown  .down  on  the  ear 
of  the  observer.  To  explain  the  result  of  the  third  trial  of  the  second 
day,  we  have  only  to  suppose  that  the  influence  of  the  upper  current 
was  less  than  that  of  the  lower.  The  conditions  for  these  observations 
were  unusually  favorable,  the  weather  continuing  the  same  during  the 
two  days,  and  the  change  of  the  wind  also  taking  place  at  nearly  the 
same  hour. 

The  fact  thus  established  is  entirely  incompatible  with  the  supposition 
that  the  diminution  in  the  sound  is  principally  caused  by  a  want  of 
homogeneity  in  the  constitution  of  the  atmosphere,  since  this  would 
operate  to  absorb  sound  equally  in  both  directions. 

In  May,  1873,  Professor  Tyndall  commenced  a  series  of  investigations 
on  the  subject  of  the  transmission  of  sound,  under  the  auspices  of  the 
Trinity  House,  of  England,  in  which  whistles,  trumpets,  guns,  and  a 
siren  were  used,  the  last-named  instrument  having  been  lent  by  the 
Light-House  Board  of  the  United  States  to  the  Trinity  House  for  the 
purpose  of  the  experiments  in  question.  The  results  of  these  investi- 
gations were,  in  most  respects,  similar  to  those  which  we  had  previously 
obtained.  In  regard  to  the  efficiency  of  the  instruments,  the  same  order 
was  determined  which  has  been  given  in  this  report,  namely,  the  siren, 
the  trumpet,  and  the  whistle.  Professor  Tyndall's  opinion  as  to  the 
efficiency  of  the  siren  may  be  gathered  from  the  following  remarks. 
Speaking  of  the  obstruction  of  sound  in  its  application  as  a  fog-signal, 
he  says,  "There  is  but  one  solution  of  this  difficulty,  which  is  to  make 
the  source  of  sound  so  powerful  as  to  be  able  to  endure  loss  and  still 
retain  sufficient  residue  for  transmission.  Of  all  the  instruments  hitherto 
examined  by  us  the  siren  comes  nearest  to  the  fulfillment  of  this  condi- 
tion, and  its  establishment  upon  our  coasts  will,  in  my  opinion,  prove  an 
incalculable  boon  to  the  mariner/'  Professor  Tyndall  arrived  at  the 
conclusions  which  the  information  we  had  collected  tended  to  establish, 
that  the  existence  of  fog,  however  dense,  does  not  materially  interfere 
with  the  propagation  of  sound ;  and  also  that  sound  is  generally  heard 
farther  with  the  wind  than  against  it,  although  the  variation  of  the  in- 


510  RESEARCHES   IN   SOUND. 

tensity  of  the  sound  is  not  in  all  cases  in  proportion  to  the  velocity  of 
the  wind.  The  result  of  his  investigations  in  regard  to  the  pitch  of  sound 
was  also  similar  to  those  we  have  given ;  and,  indeed,  all  the  facts  which 
he  has  stated  are,  with  a  single  exception  as  to  the  direction  of  the  echo, 
in  strict  accordance  with  what  we  have  repeatedly  observed.  We  regret 
to  say,  however,  that  we  cannot  subscribe  to  the  conclusions  which  he 
draws  from  his  experiments  as  to  the  cause  of  the  retardation  of  sound 
that  it  is  due  to  a  flocculent  condition  of  the  atmosphere,  caused  by  the 
intermingling  with  it  of  invisible  aqueous  vapor. 

That  a  flocculent  condition  of  the  atmosphere,  due  to  the  varying  den- 
sity produced  by  the  mingling  of  aqueous  vapor,  is  a  true  cause  of  ob- 
struction in  the  transmission  of  sotind  is  a  fact  borne  out  by  deduction 
from  the  principles  of  wave-motion,  as  well  as  by  the  experiments  of  the 
distinguished  physicist  of  the  Eoyal  Institution  of  Great  Britain ;  but 
from  all  the  observations  we  have  made  on  this  subject  we  are  far  from 
thinking  that  this  is  the  efficient  cause  of  the  phenomena  under  consider- 
ation. A  fatal  objection,  we  think,  to  the  truth  of  the  hypothesis  Pro- 
fessor Tyndall  has  advanced  is  that  the  obstruction  to  the  sound,  what- 
ever may  be  its  nature,  is  not  the  same  in  different  directions.  We  think 
we  are  warranted  in  asserting  that  in  the  cases  of  acoustic  opacity  which 
he  has  described,  if  he  had  simultaneously  made  observations  in  an  op- 
posite direction,  he  would  have  come  to  a  different  conclusion.  That  a 
flocculent  condition  of  the  atmosphere  should  slightly  obstruct  the  sound 
is  not  difficult  to  conceive ;  but  that  it  should  obstruct  the  ray  in  one 
direction  and  not  in  an  opposite,  or  in  a  greater  degree  in  one  direction 
than  in  another,  the  stratum  of  air  being  the  same  in  both  cases,  is  at 
variance  with  any  fact  in  nature  with  which  we  are  acquainted.  We 
would  hesitate  to  speak  so  decidedly  against  the  conclusions  of  Professor 
Tyndall,  for  whose  clearness  of  conception  of  physical  principles,  skill 
in  manipulation,  and  power  of  logical  deduction  we  entertain  the  highest 
appreciation,  were  the  facts  which  were  obtained  in  our  investigations 
of  a  less  explicit  character. 

While  the  phenomena  in  question  are  incompatible  with  the  assump- 
tion of  a  flocculent  atmosphere  as  a  cause,  they  are  in  strict  accordance 
with  the  hypothesis  of  the  refraction  of  the  waves  of  sound  due  to  a 
difference  in  velocity  in  the  upper  and  lower  portions  of  the  currents  of 
air.  We  do  not  say,  however,  that  the  transmission  of  sound  in  the 
atmosphere  is  fully  investigated,  or  that  the  abnormal  phenomena  which 
are  said  to  have  been  observed  in  connection  with  fog-signal  stations 
have  been  fully  explained.  So  far  from  this,  we  freely  admit  we  are  as 
yet  in  ignorance  as  to  how  the  hypothesis  we  have  adopted  is  applicable 
to  the  critical  explanation  of  the  obstruction  to  sound  in  the  abnormal 
cases  mentioned  by  General  Duane.  We  feel,  however,  considerable 
confidence  in  its  power  to  afford  a  rational  explanation  of  these  pheno- 
mena when  the  conditions  under  which  they  exist  shall  have  been  accu- 
rately determined. 


RESEARCHES   IN    SOUND.  511 

We  are  farther  confirmed  in  our  conclusion  by  the  publication  of  an 
interesting1  paper  in  the  proceedings  of  the  Royal  Society  by  Professor 
Osborne  Eeynolds,  of  Owens  College,  Manchester,  intended  to  show  that 
sound  is  not  absorbed  by  the  condition  of  the  atmosphere,  but  refracted 
in  a  manner  analogous  to  the  hypothesis  which  has  been  adopted  in  the 
preceding  report. 

Much  further  investigation  is  required  to  enable  us  to  fully  under- 
stand the  effects  of  winds  on  the  obstruction  of  sound,  and  to  determine 
the  measure  of  the  effect  of  variations  of  density  in  the  air  due  to  in- 
equality of  heat  and  moisture.  But  such  investigations  can  only  be 
made  under  peculiar  conditions  of  weather  and  favorable  localities,  with 
the  aid  of  a  number  of  steamers,  and  a  series  of  observers,  by  whom  the 
transmissibility  of  the  air  may  be  simultaneously  observed  in  different 
directions.  The  position  which  we  were  so  fortunate  to  obtain  in  our 
experiments  in  the  lower  bay  of  New  York  at  the  season  of  the  prev- 
alence of  land  and  sea  breezes  was  exceptionally  favorable  for  the  study 
of  the  action  of  wind  upon  sound.  It  is  the  intention  of  the  Light- 
House  Board  to  continue  observations  in  regard  to  this  matter,  and  to 
embrace  every  favorable  opportunity  for  their  prosecution  under  new 
and  varied  conditions. 

LIGHT-HOUSE  BOARD,  October,  1874. 


PAET  IV.— INVESTIGATIONS  IN  1875.* 
PRELIMINARY  REMARKS. 

In  the  Appendix  to  the  Light-House  Report  of  1874 1  gave  an  account 
of  a  series  of  investigations  relative  to  fog-signals,  which  had  been  made 
at  different  times  under  the  direction  of  the  chairman  of  the  committee 
on  experiments. 

These  investigations  were  not  confined  to  the  instruments  for  produc- 
ing sound,  but  included  a  series  of  observations  on  sound  itself,  in  its 
application  to  the  uses  of  the  mariner.  In  the  course  of  these  investiga- 
tions the  following  conclusions  were  early  arrived  at : 

1st.  That  the  rays  of  a  beam  of  loud  sound  do  not.  like  those  of  light, 
move  parallel  to  each  other  from  the  surface  of  a  concave  reflector,  but 
constantly  diverge  laterally  on  all  sides ;  and,  although  at  first  they  are 
more  intense  in  the  axis  of  the  reflector,  they  finally  spread  out  so  as  to 
encompass  the  whole  horizon,  thus  rendering  the  use  of  reflectors  to 
enforce  sound  for  fog-signals  of  little  value. 

2d.  That  the  effect  of  wind  in  increasing  or  diminishing  sound  is  not 
confined  to  currents  of  air  at  the  surface  of  the  earth,  but  that  those  of 
higher  strata  are  also  active  in  varying  its  transmission. 

3d.  That  although  sound  is  generally  heard  farther  with  the  wind  than 
against  it,  yet  in  some  instances  the  reverse  is  remarkably  the  case,  espec- 

*  From  tlie  Report  of  the  Light-House  Board,  for  1875. 


512  RESEARCHES   IN   SOUND. 

ially  in  one  locality,  in  which  the  sound  is  heard  against  a  northeast 
snow-storm  more  distinctly  than  when  the  wind  is  in  an  opposite  direc- 
tion. This  anomaly  was  referred  to  the  action  of  an  upper  current  in 
an  opposite  direction  to  that  at  the  earth,  such  a  current  being  known 
to  exist  in  the  ca.se  of  northeast  storms  on  our  coast.  But  in  what  man- 
ner the  action  of  the  wind  increased  or  diminished  the  audibility  of 
sound  was  a  problem  not  solved.  It  could  not  be  due,  as  might  be 
thought  at  first  sight,  to  the  acceleration  of  the  sonorous  impulse  by  the 
addition  of  the  velocity  of  the  wind  to  that  of  sound,  on  the  one  hand, 
nor  to  the  retardation  of  the  latter  by  the  motion  of  the  wind,  on  the 
other.  The  inadequacy  of  this  explanation  must  be  evident  when  we 
reflect  that  sound  moves  at  the  rate  of  750  miles  an  hour,  and  therefore 
a  wind  of  7  J  miles  an  hour  would  only  increase  its  velocity  one  per  cent,  j 
whereas  the  actual  increase  in  audibility  produced  by  a  wind  of  this 
intensity  is  in  some  instances  several  hundred  per  cent. 

In  this  state  of  our  knowledge,  a  suggestion  of  Professor  Stokes,  of 
Cambridge,  England,  which  offered  a  plausible  explanation  of  the  action 
of  the  wind,  became  known  to  us,  and  was  immediately  adopted  as  a 
working  hypothesis  to  direct  investigations. 

This  suggestion,  the  importance  of  which  appears  to  have  escaped 
general  recognition,  is  founded  on  the  fact  that  the  several  strata  into 
which  a  current  of  air  may  be  divided  do  not  move  with  the  same  veloc- 
ity. The  lower  stratum  is  retarded  by  friction  against  the  earth  and  by 
the  various  obstacles  it  meets  with,  the  one  immediately  above  by  friction 
against  the  lower,  and  so  on;  hence  the  velocity  increases  from  the  ground 
upward — a  conclusion  established  by  abundant  observation.  Now,  in 
perfectly  still  air,  a  sounding  instrument,  such  as  a  bell,  produces  a  series 
of  concentric  waves  perfectly  spherical ;  but  in  air  in  motion  the  differ- 
ence of  velocity  above  and  below  disturbs  the  spherical  form  of  the 
sound-wave,  giving  it  somewhat  the  character  of  an  oblique  ellipsoid, 
by  tending  to  flatten  it  above — to  the  windward,  and  to  increase  its  con- 
vexity above — to  the  leward ;  and  since  the  direction  of  the  sound  is  per- 
pendicular to  the  sound-wave,  against  the  wind  it  will  be  thrown  upward 
above  the  head  of  the  observer,  and  in  the  opposite  direction  downward 
toward  the  earth.  A  similar  effect  will  be  produced,  but  with  some  varia- 
tions and  perhaps  greater  intensity,  by  a  wind  above,  opposite  to  that  at 
the  surface  of  the  earth. 

These  propositions  will  be  rendered  plain  by  the  following  illustrations 
(Figures  1,  2,  and  3),  for  which  I  am  indebted  to  an  article  in  the  Ameri- 
can Journal  of  Science,  by  William  B.  Taylor. 

Fig.  l. 


RESEARCHES    IN    SOUND. 


513 


In  these,  Figure  1  represents  the  effect  of  a  favorable  wind  in  depress- 
ing the  waves  of  sound,  S  being  the  signal- station  and  O  the  point  of 
observation.  The  wind  blowing  from  W  to  E,  as  the  spheroidal  faces 
of  the  sonorous  waves  become  more  pressed  forward  by  the  greater  ve- 
locity of  the  wind  above,  assuming  it  to  be  retarded  at  the  surface  by 
friction,  and  the  direction  of  the  acoustic  beam  being  constantly  normal 
to  the  wave-surfaces,  the  lines  of  direction  of  the  sound  will  gradually 
be  bent  downward  and  reach  the  ear  of  the  observer  with  an  accumu- 
lated effect  at  the  point  O. 

Fig.  2. 


Figure  2  represents  the  ordinary  effect  of  an  opposing  wind  blowing 
from  E  to  W  against  the  sound  ;  the  wave-faces  being  more  resisted 
above  than  below,  assuming  as  before  a  retardation  at  the  surface,  the 
sound-beams  are  curved  upward,  and  the  lowest  ray  that  would  reach, 
in  still  air,  the  distant  observer  at  O,  is  gradually  so  tilted  up  that  it 
passes  above  the  ear  of  the  listener,  leaving  him  in  an  acoustic  shadow. 

Fig.  3. 


o  . 


Figure  3  represents  the  disturbing  effect  of  two  winds,  the  lower  in 
opposition  to  the  sound  at  the  surface,  and  the  upper  with  it.  In  this 
case  the  principal  effect  will  be  a  depression  of  the  sound-beam,  similar 
to  that  shown  in  Figure  1,  but  more  strongly  marked,  as  the  difference 
of  motion  will  be  greater  as  we  ascend.  Attending  this  action,  says 
Mr.  Taylor,  there  will  probably  be  some  lagging  of  the  lower  stratum 
by  reason  of  the  surface-friction,  the  tendency  of  which  will  be  to  dis- 
tort the  lower  part  of  the  sound-waves,  giving  them  a  reverse  or  serpen- 
tine curvature.  In  this  case  the  upper  ray  of  sound  would  only  have  a 
single  curvature,  similar  to  that  shown  in  Figure  1,  while  the  lower  rays 
would  be  represented  by  the  lower  line  S  O,  rendering  the  sound  less 
audible  at  an  intermediate  point,  £,  than  at  the  more  distant  station  O. 
This  hypothetical  case  of  compound  refraction  offers  a  plausible  ex- 
planation of  the  paradox  of  a  nearer  sound  being  diminished  in  power 
by  the  wind  which  increases  the  effect  of  a  more  distant  one. 
S.  Mis.  59 33 


514  EESEARCHES    IN   SOUND. 

In  these  figures  and  all  the  succeeding  ones  the  direction  of  the  wind 
is  indicated  by  arrows. 

The  hypothesis  we  have  adopted  in  connection  with  the  fact  of  the 
lateral  spread  of  sound  gives  a  simple  explanation  of  various  abnormal 
phenomena  of  sound  such  as  has  been  observed  in  the  previous  investi- 
gations, and  of  which  the  following  are  examples :  First,  the  audibility 
of  a  sound  at  a  distance,  and  its  inaudibility  nearer  the  source  of  sound ; 
second,  the  inaudibility  of  a  sound  at  a  given  distance  in  one  direction, 
while  a  lesser  sound  is  heard  at  the  same  distance  in  an  opposite  direc- 
tion ;  third,  the  audibility  of  the  sound  of  an  instrument  at  one  time 
at  the  distance  of  several  miles,  while  at  another  time  the  sound  of  the 
same  instrument  cannot  be  heard  at  more  than  a  fifth  of  the  same  dis- 
tance ;  fourth,  while  the  sound  is  heard  generally  farther  with  the  wind 
than  against  it,  in  some  instances  the  reverse  is  the  case  $  fifth,  the  sud- 
den loss  of  sound  in  passing  from  one  locality  to  another  in  the  same 
vicinity,  the  distance  from  the  source  of  the  sound  being  the  same. 

The  first  four  of  these  phenomena  find  a  ready  explanation  in  the 
hypothesis  adopted  by  supposing  an  increase  or  diminution  in  the  rela- 
tive velocity  of  the  currents  of  wind  in  the  upper  or  lower  strata  of  air. 
The  fifth  is  explained  by  the  interposition  of  an  obstacle  which  casts, 
as  it  were,  a  sound-shadow,  disappearing  at  a  given  distance  by  the 
divergence  of  the  rays  on  each  side  of  the  obstacle  into  what  would  be 
an  optical  shadow. 

Accounts  of  these  investigations  were  presented  from  time  to  time  to 
the  Light-House  Board,  and  to  the  Philosophical  Society  of  Washington 
in  1872.  Subsequently  a  series  of  investigations  on  the  same  subject 
was  instituted  in  England  by  the  Elder  Brethren  of  the  Trinity  House, 
under  the  direction  of  their  scientific  adviser,  the  celebrated  physicist, 
Dr.  Tyndall.  While  in  the  latter  investigations  various  abnormal  phe- 
nomena, similar  in  most  instances  to  those  we  have  mentioned,  were 
observed,  they  were  referred  by  Dr.  Tyndall  to  an  entirely  different 
cause,  viz,  to  the  existence  of  acoustic  clouds,  consisting  of  portions  of 
the  atmosphere  in  a  flocculent  or  mottled  condition,  due  to  the  unequal 
distribution  of  heat  and  moisture,  which,  absorbing  and  reflecting  the 
sound,  produce  an  atmosphere  of  acoustic  opacity.  While  we  do  not 
deny  the  possible  existence  of  such  a  condition  of  the  atmosphere,  we 
think  it  insufficient  to  account  for  all  the  phenomena  in  question,  and 
believe  that  a  more  general  and  efficient  cause  is  that  of  the  wind,  in 
accordance  with  the  hypothesis  of  Professor  Stokes. 

We  regret  to  differ  in  opinion  from  Dr.  Tyndall,  and  have  published 
our  dissent  from  his  views  in  no  spirit  of  captious  criticism  or  desire  to 
undervalue  the  results  he  has  obtained,  some  of  which  are  highly  im- 
portant. Our  only  object  in  our  remarks  and  in  our  investigations  is 
the  establishment  of  truth. 

The  determination  of  the  question  as  to  the  cause  of  the  abnormal 
phenomena  of  sound  we  have  mentioned,  and  the  discovery  of  new  phe 


KESEAECHES   IN   SOUND.  515 

noinena,  are  not  mere  matters  of  abstract  scientific  interest,  but  are  of 
great  practical  importance,  involving  the  security  of  life  and  property, 
since  they  include  the  knowledge  necessary  to  -the  proper  placing  of 
fog-signals,  and  the  instruction  of  mariners  in  the  manner  of  using 
them. 

The  hypothesis  we  have  adopted,  that  of  the  change  of  direction  of 
sound  by  the  unequal  action  of  the  wind  upon  the  sound-waves,  is 
founded  on  well-established  mechanical  principles,  and  offers  a  ready 
explanation  of  facts  otherwise  inexplicable.  It  is  also  a  fruitful  source 
from  which  to  deduce  new  consequences  to  be  verified  or  disproved  by 
direct  experiment.  It  would  however  ill  become  the  spirit  of  true 
science  to  assert  that  this  hypothesis  is  sufficient  to  explain  all  the  facts 
which  may  be  discovered  in  regard  to  sound  in  its  application  to  fog- 
signals,  or  to  rest  satisfied  with  the  idea  that  no  other  expression  of  a 
general  principle  is  necessary.  An  investigation  however  to  be  fruit- 
ful in  results,  as  a  general  rule,  must  be  guided  by  a  priori  conceptions. 
Hap-hazard  experiments  and  observations  may  lead  to  the  discovery  of 
isolated  facts,  but  rarely  to  the  establishment  of  scientific  principles. 
There  is  danger  however  in  the  use  of  hypotheses,  particularly  by  those 
inexperienced  in  scientific  investigations,  that  the  value  of  certain 
results  may  be  overestimated,  while  to  others  is  assigned  less  weight 
than  really  belongs  to  them.  This  tendency  must  be  guarded  against. 
The  condition  of  the  experiment  must  be  faithfully  narrated,  and  a  scru- 
pulously truthful  account  of  the  results  given.  While  we  have  used 
the  hypothesis  above  mentioned  in  the  following  investigations  as  some- 
thing more  than  an  antecedent  probability,  we  have  not  excluded  ob- 
servations which  may  militate  against  it,  and  we  hold  ourselves  ready 
to  admit  the  application  of  other  principles,  or  to  modify  our  concep- 
tion of  those  we  have  adopted,  when  new  facts  are  discovered  which 
warrant  such  changes.  But  we  require  positive  evidence,  and  cannot 
adopt  any  conclusions  which  we  think  are  not  based  upon  a  logical 
correlation  of  facts. 

The  investigations  described  in  the  following  account,  though  simple 
in  their  conception,  have  been  difficult  and  laborious  in  their  execution. 
To  be  of  the  greatest  practical  value  they  were  required  to  be  made  on 
the  ocean,  under  the  conditions  in  which  the  results  are  to  be  applied  to 
the  use  of  the  mariner,  and  therefore  they  could  only  be  conducted  by 
means  of  steam-vessels  of  sufficient  power  to  withstand  the  force  of 
i  ough  seas,  and  at  times  when  these  vessels  could  be  spared  from  other 
duty.  They  also  required  a  number  of  intelligent  assistants  skilled  in 
observation  and  faithful  in  recording  results. 

OBSERVATIONS  IN  AUGUST,  1875,  AT  BLOCK  ISLAND. 

The  party  engaged  in  these  investigations  consisted  of  the  chairman 
of  the  Light-House  Board,-  General  Woodruff,  U.  S.  A.,  engineer  third 
light-house  district;  Dr.  James  C.  Welling,  president  of  Columbian  Uni« 


516  RESEARCHES    IX    SOUND. 

versify,  Washington,  D.  •  C.;  Mr.  T.  Brown,  of  New  York,  patentee  of 
the  siren;  Mr.  Edw.  Woodruff,  assistant  superintendent  of  construction ; 
and  Captain  Keeney,  Commander  of  the  light-house  steamer  Mistletoe. 
They  arrived  at  Block  Island  on  the  afternoon  of  the  4th  of  August, 
1875.  This  place  was  chosen  as  the  site  of  the  experiments,  first,  on 
account  of  its  insular  position,  being  as  it  were  in  the  prolongation  of 
the  axis  of  Long  Island,  distant  fifteen  miles  from  the  most  easterly 
part  of  the  latter,  and  entirely  exposed  to  the  winds  and  waves  of  the 
Atlantic  Ocean;  and,  secondly,  because  there  are  on  Block  Island  two 
light-houses,  one  of  which  is  of  the  first  order,  and  connected  with  it  are 
two  fog-signals,  one  of  them  with  the  latest  improvements.  (See  Fig.  4.) 

OBSERVATIONS  IN  REGARD  TO  THE  AERIAL  ECHO. 

This  phenomenon  has  been  frequently  observed  in  the  researches  of 
the  Light-House  Board,  in  case  of  powerful  sounds  from  the  siren  and 
from  the  fog-trumpet.*  It  consists  of  a  distinct  reflection  of  sound  as  if 
from  a  point  near  the  horizon  in  the  prolongation  of  the  axis  of  the  trum- 
pet. The  question  of  the  origin  of  this  echo  has  an  important  bearing, 
according  to  Dr.  Tyndall,  on  the  explanation  of  the  abnormal  phenom- 
ena of  sound  we  have  mentioned.  He  refers  it  to  the  non-homogeneous 
condition  of  portions  of  the  air,  which  reflect  back  the  waves  of  sound 
in  accordance  with  the  analogy  of  the  reflection  of  light  at  the  common 
surface  of  two  media  of  different  densities.  We  have  adopted,  as  a  pro- 
visional hypothesis,  that  it  is  due  to  the  reflection  from  the  waves  and 
the  larger  undulations  of  the  surface  of  the  ocean,  in  connection  with 
the  divergency  of  beams  of  powerful  sounds.  To  bring  these  hypotheses 
to  the  test  of  a  crucial  experiment,  arrangements  were  made,  under  the 
direction  of  Mr.  Brown,  to  change  the  direction  of  the  axis  of  one  of  the 
sirens  from  the  horizontal  to  the  vertical  position. 

The  first  observations  were  made  August  5,  with  the  siren  in  its  usual 
horizontal  position,  while  the  air  was  so  charged  with  fog  as  to  render 
the  sound  of  the  instrument  necessary  for  the  guidance  of  the  mariner, 
the  image  of  the  sun  being  obscured  and  the  land  invisible,  from  the 
sea.  Under  these  conditions  an  echo  was  heard  when  the  pressure  of  the 
steam  reached  50  pounds  per  square  inch.  The  reflection  in  this  case, 
as  usual,  was  from  a  point  in  the  sea-horizon  in  the  prolongation  of  the 
axis  of  the  trumpet.  It  was  not,  however,  heard  more  distinctly  when 
standing  near  the  origin  of  the  sound  than  at  several  hundred  feet  on 
either  side  of  it.  The  interval  between  the  cessation  of  the  original 
sound  and  the  commencement  of  the  echo  was  not  as  marked  as  in  some 
previous  observations,  not  being  more  than  four  or  five  seconds.  The 

*  The  same  phenomenon  is  mentioned  by  Froissart  in  his  account  of  the  embarkation 
of  the  expedition  of  the  French  and  English  to  the  coast  of  Africa  to  assist  the  Genoese 
against  the  pirates  in  1390.  "  It  was  a  beautiful  sight/'  says  the  chronicler,  "  to  view 
this  fleet,  with  the  emblazoned  banners  of  the  different  lords  fluttering  in  the  wind, 
and  to  hear  the  minstrels  and  .other  musicians  sounding  their  pipes,  clarions,  and  trum- 
pets, whose  sounds  were  re-echoed  back  by  the  sea."  (See  Illustrations  of  Froissart 
by  H.  N.  Humphrey,  Plate  IV.) 


RESEARCHES    IN    SOUND.  517 

duration  of  the  echo  was  on  the  average  about  eight  seconds,  beginning 
with  the  time  of  its  first  perception,  and  not  with  the  cessation  of  the 
sound  of  the  trumpet.  General  Woodruff  and  Doctor  Welling  both  noted 
the  peculiar  character  of  the  echo,  which  was  that  of  a  series  of  reflec- 
tions varying  in  intensity  from  a  maximum,  near  the  beginning,  and 
gradually  dying  away.  The  wind  was  nearly  at  right  angles  to  the  axis 
oi  the  trumpet  and  also  to  that  of  the  crests  of  the  swell  of  the  ocean, 
which  was  rolling  in  from  the  effects  of  a  commotion  without.  The 
barometer  at  12  M.  indicated  30.2  inches;  the  dry-bulb  thermometer 
73°  F.  the  wet-bulb  70°  F.  indicating  a  remarkable  degree  of  aqueous 
saturation.  During  the  whole  day  the  air  in  all  the  region  around 
Block  Island  was  undoubtedly  in  a  homogeneous  condition. 

August  6. — On  this  day  the  weather  was  nearly  the  same.  The  fog- 
signal  on  the  5th  instant  was  kept  in  operation  for  the  use  of  the  mariner 
nineteen  hours,  and  on  this  day  it  was  blown  twenty  hours  continuously. 
The  barometer  marked  30.20  inches ;  the  thermometer  70°  F. ;  the  fog- 
not  as  equally  distributed  as  on  the  preceding  day ;  the  north  end  of  the 
island,  distant  four  miles,  being  distinctly  visible.  The  wind  was  S.  W. 
to  S.,  making  an  angle  of  about  60°  with  the  axis  of  the  fog-trumpet. 
The  echo  continued  to  be  heard  distinctly  with  a  sound  varying  in 
intensity,  but  was  not  as  loud  as  we  have  heard  it  on  certain  occasions 
in  previous  years. 

During  this  and  the  preceding  day,  workmen  were  employed  .under 
Mr.  Brown  in  inserting  a  flexible  India-rubber  tube,  two  inches  in  diam- 
eter, between  the  revolving  plate  of  the  siren  and  the  smaller  end  of  the 
trumpet,  so  that  it  might  be  brought  into  a  vertical  position.  This  work, 
though  apparently  simple,  was  difficult  in  execution,  since  it  involved 
the  necessity  of  strong  supports  for  the  cast-iron  trumpet,  which  in  itself 
weighed  eight  hundred  pounds,  and  also  of  a  union  of  the  parts  of  suf- 
ficient strength  to  resist  the  pressure  of  the  steam  at  fifty  pounds  to  the 
square  inch. 

August  7. — Wind  from  the  S.  S.  W.  Fog  continued;  the  workmen  had 
not  as  yet  completed  the  attachment. 

August  9. — Barometer  30.30  inches  at  12  M.  Dry -bulb  thermometer 
740  F.;  wet  bulb  71°.5.  Wind  S.  S.  W.  Fog  dense  along  the  south 
coast,  but  light  over  all  the  northern  portion  of  the  island.  The  echo 
was  heard  all  day,  not  very  loudly,  but  distinctly.  Siren  still  horizontal, 
the  arrangement  for  elevating  it  not  having  been,  at  10  a.  m.,  completed. 
Experiments  were  made  on  the  reciprocal  sounds  of  the  whistles  from 
two  steamers,  the  results  to  be  given  hereafter.  At  5  p.  M.  the  adjust- 
ment of  the  flexible  tube  to  the  smaller  end  of  the  trumpet  was  finished, 
which,  giving  an  additional  length  to  the  instrument  of  about  5  feet, 
threw  it  out  of  unison  with  the  siren  proper.  To  restore  this  unison  the 
speed  of  revolution  of  the  perforated  plate  was  diminished,  and  after  this 
the  trumpet,  still  being  horizontal,  was  sounded.  An  echo  similar  in 


518  RESEARCHES    IN    SOUND. 

character  to  those  which  had  been  observed  011  the  preceding  day,  and 
the  earlier  part  of  the  same  day,  was  produced. 

August  10. — Barometer  30.10  inches.  Dry  bulb  74° ;  wet  bulb  69°  F. 
Wind  W.  S.  W.  5  atmosphere  hazy.  Observations  first  made  with  the 
trumpet  horizontal.  Echo  as  that  of  preceding  days,  distinct  but  not 
very  loud,  and  coming  principally  from  the  portion  of  the  horizon  in  the 
direction  of  the  axis  of  the  trumpet.  The  position  of  the  trumpet  was 
then  changed,  its  axis  being  turned  to  the  zenith  in  order  to  make  what 
was  thought  might  be  a  crucial  experiment.  When  the  trumpet  was 
now  sounded  a  much  louder  echo  was  produced  than  that  which  was 
heard  with  the  axis  of  the  trumpet  horizontal,  and  it  appeared  to  encircle 
the  whole  horizon  $  but  though  special  attention  was  directed  to  the  point 
by  all  the  party  present,  no  reverberation  was  heard  from  the  zenith. 
The  echo  appeared  however  to  be  more  regular  and  prolonged  from  the 
ocean  portion  of  the  horizon  than  from  that  of  the  land. 

In  this  experiment,  while  there  was  no  reflection  from  the  zenith  in 
which  the  sonorous  impulse  was  strongest,  there  must  have  been 
reverberations  from  the  surface  of  the  land  and  the  ocean.  This  will 
be  evident  when  we  consider  the  great  divergency  of  sound  by  which 
sonorous  waves  from  a  vertical  trumpet  are  thrown  down  to  the 
plane  of  the  horizon  on  every  side,  some  of  which,  meeting  oblique  sur- 
faces, must  be  reflected  back  to  the  ear  of  the  observer  near  the  source 
of  the.  sound.  This  inference  will  be  more  evident  when  it  is  recol- 
lected that  the  reflected  rays  of  sound  diverge  as  well  as  those  of  the 
original  impulse.  Hence  reflection  from  the  surface  of  the  sea  is  a  true 
cause  of  the  echo,  but  whether  it  be  a  sufficient  one  may  require  further 
investigation.  For  this  explanation  it  is  not  necessary  that  the  sea 
should  be  covered  with  crested  waves ;  a  similar  effect  would  take  place 
were  the  surface  perfectly  smooth  but  in  the  form  of  wide  swells,  swhich 
in  places  exposed  to  an  open  sea  are  scarcely  ever  absent.  Moreover, 
the  increased  loudness  of  the  echo  is  a  fact  in  accordance  with  the  same 
view. 

The  observations  were  repeated  with  the  same  effect  on  succeeding 
days,  until  this  class  of  experiments  was  ended  by  the  bursting  of  the 
India-rubber  tube.  Had  a  distinct  echo  been  heard  from  the  zenith,  the 
result  would -have  been  decidedly  in  favor  of  the  hypothesis  of  a  reflec- 
tion from  the  air ;  but  as  this  was  not  the  case  the  question  still  remained 
undetermined,  especially  since  the  atmosphere  during  these  experiments 
was  evidently  in  a  homogeneous  condition.  We  do  not  agree  however 
in  the  position  taken  in  the  report  of  the  Trinity  Board,  that  on  the 
origin  of  this  echo  depends  the  whole  solution  of  the  problem  as  to  the 
efficient  cause  of  the  abnormal  phenomena  of  sound.  The  ingenious  ex- 
perimental illustrations  of  the  reflection  of  sound  from  a  flame  or  heated 
air,  establish  clearly  the  possibility  of  such  reflection;  but  it  must  be 
remembered  they  were  made  under  exaggerated  conditions,  the  atmos- 
phere being  in  a  state  of  extreme  rarefaction  in  a  limited  space,  and  the 


RESEARCHES    IN    SOUND.  519 

sound  of  a  feeble  character,  while  the  phenomena  in  nature  are  produced 
with  a  comparatively  small  difference  of  temperature  and'with  powerful 
sounds. 

EXPERIMENTS  AT  BLOCK  ISLAND 

RELATIVE  TO  THE  EFFECT  OF  ELEVATION  ON  AUDIBILITY. 

For  this  investigation  the  first-order  light-house  at  Block  Island  offered 
peculiar  facilities.  It  is  situated  near  the  edge  of  a  perpendicular  blufft 
152  feet  above  the  sea.  The  tower  being  52  feet  above  the  base,  gives  a 
total  height  to  the  focal  plane  of  the  lens  of  204  feet,  on  the  level  of  which 
the  ear  of  the  observer  could  be  placed. 

The  first  and  second  experiments  of  this  class  were  made  on  the  10th 
of  August,  with  two  light-house  steamers,  the  Putnam  and  the  Mistletoe, 
moving  simultaneously  in  opposite  directions.  The  barometer  indicated 
30.10  inches  of  atmospheric  pressure ;  the  dry-bulb  thermometer  indi- 
cating 74°  F.,  and  the  wet-bulb  69°.  The  wind  at  the  time  of  the  ex- 
periments was  from  the  west,  and  of  a  velocity  of  seven  miles  per  hour. 
The  vessels  started  from  the  point  C,  Fig.  4,  opposite  the  light-house, 
A,  about  one  mile  distant,  a  position  as  near  the  shore  as  it  was  con- 
sidered safe  to  venture.  The  Putnam  steamed  with  the  wind,  the  Mis- 
tletoe steamed  against  the  wind,  each  blowing  its  whistle  every  half 
minute.  The  duration  of  the  sound  was  noted  at  the  top  of  the  tower  and 
at  the  level  of  the  sea,  Mr.  Brown  being  the  observer  at  the  latter  sta- 
tion, while  the  chairman  of  the  board,  with  an  assistant,  observed  at  the 
former.  On  comparing  notes,  the  watches  having  been  previously  set 
to  the  same  time,  it  was  found — 

First.  That  the  duration  of  the  sound  on  the  tower,  when  coming 
against  the  wind,  was  nine  minutes,  while  at  the  base  of  the  cliff  it  was 
heard  only  one  minute.  It  was  afterward  found  from  the  records  on 
board  of  the  Putnam,  the  sound  of  which  came  against  the  wind,  that 
this  vessel  was  moving,  during  the  experiment,  at  half-speed,  and  hence 
the  duration  of  the  sound  on  the  tower  should  be  considered  as  4£  minutes, 
and  the  difference  in  favor  of  audition  on  the  tower  4  minutes  instead 
of  8,  as  given  by  the  first  record. 

Second.  That  the  sound  of  the  Mistletoe,  coming  to  the  observer  with 
the  wind,  was  heard  on  the  tower  during  15  minutes,  while  it  was  heard 
at  the  base  of  the  cliff  during  34  minutes,  the  difference  being  19  minutes 
in  favor  of  hearing  at  the  level  of  the  sea.  This  result,  which  differs  from 
that  of  all  the  other  experiments  of  the  same  class,  deserves  special  at- 
tention. 

After  making  the  foregoing  experiments  of  this  class,  and  others,  on 
the  effect  of  wind  on  sound,  to  be  described  in  the  next  section,  the  ves- 
sels were  called  off  for  other  duty,  and  the  investigations  were  not  re- 
sumed until  August  17,  when  the  following  experiments  were  made : 

The  third  experiment. — The  wind  was  from  the  E.  ~S.  E.,  at  the  rate  of 


520 


RESEARCHES   IN   SOUND. 


about  five  miles  per  hour  at  the  surface,  and  a  greater  velocity  at  the 
height  of  the  tower.    Barometer,  30.25  ins. ;  thermometer,  72°. 

Fig.  4. 


BLOCK    ISLAND 


L.  9    Buoy 


:LH. 


O  1 


Scale  of  miles 
2  3 


RESEARCHES   IN   SOUND.  521 

In  tliis  and  the  subsequent  experiments  of  the  same  day  but  one 
steamer— the  Mistletoe — was  employed.  It  started  at  10:30  A.  M.  from 
the  point  C,  Fig.  4,  at  the  foot  of  the  cliff,  and  steamed  W.  S.  W.  along 
C  B  for  about  12  minutes,  or  a  distance  of  two  miles,  blowing  the  whistle 
every  half-minute.  To  note  the  duration  of  the  sound,  Dr.  Welling  was 
stationed  at  the  foot  of  the  cliff,  at  the  level  of  the  sea,  while  the  chair- 
man of  the  Light-House  Board,  with  an  assistant  who  acted  as  clerk, 
was  on  the  upper  gallery  of  the  tower,  the  ears  of  the  latter  being  almost 
precisely  200  feet  above  those  of  the  observer  at  the  foot  of  the  cliff. 

The  watches  having  been  previously  set  to  the  same  time,  on  compar- 
ing results  it  was  found  that  the  whistle  was  heard  at  the  top  of  the 
tower  for  twelve  minutes  and  at  the  bottom  of  the  cliff  for  five  and  one- 
half  minutes,  making  the  difference  in  favor  of  audition  on  the  tower  six 
and  one-half  minutes.  In  this  experiment  the  sound  came  to  the  ob- 
servers nearly  against  the  wind. 

The  fourth  experiment  consisted  in  directing  the  Mistletoe  to  proceed 
in  the  opposite  direction  from  the  same  point,  along  the  line  C  D.  It 
started  at  11:5  A.  M.  the  breeze  being  light  at  the  time,  and  proceeded 
about  two  and  one-half  miles  before  the  sound  was  lost  to  the  observers. 
On  comparing  notes  it  was  found  that  the  sound  was  heard  at  the  top 
of  the  tower  during  fifteen  minutes,  and  at  the  level  of  the  sea  for  eleven 
minutes,  giving  a  difference  in  favor  of  the  hearing  on  the  top  of  the 
tower  of  four  minutes. 

In  the  fifth  experiment,  the  Mistletoe  steamed  again  in  the  Direction 
with  the  wind,  the  sound  from  its  whistle  coming  to  the  ears  of  the  ob- 
servers against  the  wind.  Starting  about  11:45  A.  M.  and  steaming  about 
two  miles,  the  sound  was  heard  on  the  tower  during  twelve  minutes 
and  at  the  foot  of  the  cliff'  during  five  and  one-half  minutes,  making  a 
difference  of  six  and  a  half  minutes  in  favor  of  audition  on  the  tower. 
Previous  to  this  experiment  the  wind  had  veered  one  point  to  the  west, 
bringing  the  direction  of  the  sound  to  the  observers  in  less  direct  oppo- 
sition to  the  wind  than  in  the  last  experiment. 

The  siyth  experiment. — In  this  case  the  steamer  was  directed  to  proceed> 
in  the  opposite  direction,  or  against  the  wind,  so  that  the  sound  of  the 
whistle  would  reach  the  ear  of  the  observers  in  the  same  direction  as 
that  of  the  wind.  It  started  at  12:19  p.  M.  and  proceeded  two  and  one- 
sixth  miles  ;  the  whistle  was  heard  during  thirteen  minutes  on  the  top  of 
the  tower,  and  at  the  bottom  of  the  cliff  during  precisely  the  same  time, 
the  difference  between  the  top  of  the  tower  and  the  bottom  of  the  cliff 
in  this  case  being  nothing. 

The  vessel  having  again  been  called  off  on  other  duty  the  seventh  ex- 
periment was  made  the  1st  of  September.  On  this  day  the  wind  was 
northeast ;  the  velocity  at  the  top  of  the  tower  was  thirteen  and  a  half 
miles  per  hour,  and  at  the  bottom  of  the  tower  eleven  miles  per  hour. 
The  barometer  indicated  30.20  inches  pressure,  the  dry  bulb  72°,  and  the 
wet  bulb  G7o. 


522  RESEARCHES    IN    SOUXD. 

The  theoretical  conditions  for  exhibiting  the  effect  of  height  on  audi- 
tion in  this  experiment  were  much  more  favorable  thah  any  of  the  pre- 
ceding. First,  the  velocity  of  the  wind  was  greater ;  second,  the  differ- 
ence between  the  velocities  at  top  and  bottom  of  the  tower  was  well 
marked,  and  the  direction  of  the  wind  was  more  favorable  for  direct 
opposition  to  the  sound  as  it  came  to  the  ear  of  the  observer.  In  this 
case  General  Woodruff  was  the  observer  at  the  bottom  of  the  cliff,  while 
the  chairman  of  the  Light-House  Board  and  his  assistant,  with  several 
visitors,  were  at  the  top  of  the  tower. 

The  steamer  started  at  10:58  A.  M.  and  proceeded  during  eight  minutes, 
or  a  mile  and  one- third,  when  the  sound  was  lost  at  the  top  of  the  tower. 
In  this  case,  though  the  sound  was  heard  for  eight  minutes  from  the  top 
of  the  tower,  and  the  first  five  blasts  marked  on  the  notes  as  quite  loud, 
it  was  not  heard  at  all  at  the  bottom  of  the  cliff  at  least  a  hundred  yards 
nearer  the  source  of  the  sound. 

This  result,  which  interested  and  surprised  a  number  of  intelligent 
visitors,  who  were  in  the  tower  at  the  time,  strikingly  illustrates  the 
effect  of  elevation  on  the  audibility  of  sound  moving  against  the  wind. 
The  result  was  so  important  that  it  was  thought  advisable  to  imme- 
diately repeat  the  experiment  under  the  same  conditions. 

In  the  eighth  experiment,  the  Mistletoe  was  again  directed  to  proceed^ 
in  the  direction  of  the  wind,  along  the  line  it  had  previously  traversed. 
It  started  at  11:25  A.  M.,  and  proceeded  during  six  minutes,  or  one  mile, 
when  the  sound  was  lost  at  the  top  of  the  tower.  In  this  case,  the  first 
blast  of  the  whistle  was  feebly  heard  at  the  base  of  the  cliff,  but  no 
other,  while  thirteen  blasts  were  heard  at  the  top  of  the  tower,  of  which 
the  first  six  were  marked  as  loud. 

That  this  remarkable  effect  was  not  produced  by  an  acoustic  cloud  or 
a  flocculent  atmosphere  is  evident  from  the  experiment  which  imme- 
diately succeeded. 

The  ninth  experiment. — In  this  trial,  the  Mistletoe  was  directed  to  pro- 
ceed against  the  wind,  so  that  the  sound  of  its  whistle  should  come  to 
the  ears  of  the  observers  with  the  wind.  It  started  at  11:48  A.  M.,  and 
proceeded  during  sixteen  minutes,  or  two  and  two-thirds  miles,  when 
the  sound  of  its  whistle  was  lost  to  the  observers  on  the  top  of  the  tower. 
In  this  case  the  sound  of  the  whistle  became  audible  at  the  bottom  of 
the  cliff  as  soon  as  the  position  of  the  vessel  became  such  as  to  bring  the 
sound  to  the  observers  approximately  with  the  wind,  and  continued  to 
be  audible  during  fifteen  minutes,  or  within  one  minute  as  long  as  the 
sound  was  heard  at  the  top  of  the  tower. 

It  may  be  mentioned  as  an  interesting  fact,  that  an  assistant  who  was 
observing  the  sound  with  General  Woodruff  at  the  foot  of  the  cliff,  when 
the  sound  could  not  be  heard  at  the  level  of  the  sea,  in  the  sixth  experi- 
ment perceived  it  distinctly  by  ascending  the  side  of  the  cliff  to  a  height 
of  twenty-five  or  thirty  feet. 

All  the  conditions  and  results  of  these  experiments  are  strikingly  in 


RESEARCHES    IN    SOUND. 


523 


conformity  with  the  theory  of  the  refraction  of  sound  which  we  have  pre- 
viously explained. 

The  following  recapitulation  of  the  results  of  the  foregoing  experi- 
ments will  exhibit  their  correspondence  with  the  general  theory : 

Sound  heard  coming  against  the  wind. 


Experiments. 

Duration  at  the 
top  of  the  tower. 

Duration  at  the  base  of 
the  cliff. 

Difference  in  favor  of 
audition  on  the  tower. 

First 

44-  minutes 

•i    minute 

4      minutes. 

Third 

12    minutes 

54-    minutes 

&J    minutes. 

Fifth.   ... 

12    minutes 

5^    minutes  .- 

6^    minutes. 

Seventh. 

8    minutes 

Not  heard  

8     minutes. 

Eighth 

6    minutes 

First  blast  heard,  but 

5£    minutes. 

Average 

42£  minutes  
&J  minutes  . 

no  other, 
£    minute  after  starting. 

12     minutes. 

30£    minutes. 
6.  1  minutes. 

Sound  heard  coming  with  the  icind. 


Experiments. 

Duration  at  the 
top  of  the  tower. 

Duration  at  the  base  of 
the  cliff. 

Difference  in  favor  of 
audition  at  base  of  cliff. 

Second 

15    minutes 

34    minutes 

19    minutes 

Fourth  .  . 

]  5    minutes  .  .  

11    minutes  . 

—  4    minutes. 

Sixth 

13    minutes  ..... 

13    minutes. 

0    minutes. 

Ninth 

16    minutes 

15    minutes 

—  1    minute. 

59    minutes  .  . 

73    minutes  

14    minutes. 

Average 

14£  minutes 

18^  minutes 

34-  minutes 

From  the  first  of  the  foregoing  tables  it  appears  that  the  elevation  of 
the  observer  has  a  marked  effect  on  the  audition  of  sound  moving  against 
the  wind  while,  from  the  second,  with  one  important  exception,  it  has 
very  little,  if  any,  effect  on  sound  moving  with  the  wind.  Another  ex- 
periment relative  to  the  same  class  of  phenomena  was  made  on  the  19th 
of  August  (see  Fig.  4),  the  wind  being  S.  S.  W.  Two  observers,  General 
Woodruff  and  Dr.  Welling,  starting  from  the  bottom  of  the  cliff  imme- 
diately below7  the  light-house,  went  along  the  beach,  the  one  in  the  direc- 
tion A  /,  and  the  other  in  direction  A  e.  General  Woodruff  found  that 
the  sound  of  the  siren  was  distinctly  heard  all  the  way  to  the  break- 
water, and  was  so  loud  that  it  probably  could  have  been  heard  for  sev- 
eral miles  in  that  direction.  Dr.  Welling,  on  the  contrary,  entirely  lost 
the  sound  within  a  quarter  of  a  mile  of  the  light-house.  This  result  is 
readily  explained  as  a  case  of  lateral  refraction 5  the  wind  was  in  the 
direction  traversed  by  General  Woodruff,  and  contrary  to  that  pursued 
by  Dr.  Welling.  In  the  one  case  the  wind,  retarded  by  the  surface  of 


524  RESEARCHES    IN    SOUND. 

the  cliff,  moved  with  less  velocity  than  it  did  farther  out,  and  conse- 
quently the  sound  was 'thrown  against  the  face  of  the  cliff,  and  on  the 
ear  of  the  observer,  and  in  the  other  thrown  from  it,  thus  leaving,  as  it 
were,  a  vacuum,  of  sound.  The  effect  in  the  case  was  very  striking,  since 
the  siren  was  pointed  toward  the  zenith,  and  the  sound  in  still  air  could 
have  been  heard  for  miles  in  every  direction. 

INVESTIGATIONS  AT  BLOCK  ISLAND 

IN   REGARD   TO   THE  EFFECT   OF  WIND   ON  AUDIBILITY. 

These  were  made  by  the  aid  of  two  steamers.  Captain  Walker,  naval 
secretary  of  the  board,  having  completed  a  series  of  inspections  in  the 
Third  District,  sent  the  steamer  Putnam,  under  Captain  Fields,  to  aid 
the  Mistletoe  in  the  investigations.  They  were  commenced  on  the  9th 
of  August,  at  12  o'clock.  The  wind  was  S.  S.  W.  with  a  velocity  of  7J 
miles  per  hour.  Barometer,  30.3  inches  5  thermometer,  dry  bulb,  74°  F. 
wet  bulb,  71J°  F. 

The  two  steamers  started  from  a  buoy  near  the  north  end  of  the  island, 
the  one  steaming  against  the  wind,  and  the  other  with  it,  each  blowing  its 
whistle  every  minute.  The  distance  travelled  by  each  steamer  was  esti- 
mated by  the  running  time,  which,  from  previous  observations,  was  found 
to  be  ten  miles  per  hour.  Each  vessel  was  furnished  with  a  whistle  of 
the  same  size,  of  6  inches  diameter,  actuated  by  the  same  pressure  of  20 
pounds  of  steam,  and  which,  by  previous  comparison,  had  been  found  to 
give  sound  at  this  pressure  of  the  same  penetrating  power.  The  obser- 
vations on  the  Mistletoe  were  made  by  General  Woodruff,  and  on  the 
Putnam  by  Dr.  Welling,  each  assisted  by  the  officers  of  the  respective 
vessels.  The  two  steamers  proceeded  to  buoy  off  the  north  end  of  the 
island,  in  which  position  the  wind  was  unobstructed  by  the  land — a'low 
beach.  Indeed,  the  island  being  entirely  destitute  of  trees,  and  consist- 
ing of  a  rolling  surface,  the  wind  had  full  sweep  over  it  in  every  direction. 

First  experiment. — The  Putnam  went  against  the  wind  and  the  Mistle- 
toe in  the  opposite  direction.  The  Putnam  lost  the  sound  of  the  whistle 
of  the  Mistletoe  in  two  minutes  and  stopped,  but  continued  to  blow  the 
whistle.  The  Mistletoe  continued  on  her  course  and  heard  the  Putnam's 
whistle  for  twenty  minutes  in  all.  During  the  first  two  minutes  both 
vessels  were  in  motion,  and  therefore  the  space  through  which  the  sound 
was  heard  moving  against  the  wind  would  be  represented  by  4,  while  the 
space  through  which  the  sound  was  heard  moving  with  the  wind  would 
be  represented  by  20 •+  2  =  (22),  the  ratio  being  1 :  5J. 

Second  experiment. — In  this  the  Putnam  went  with  the  wind  and  the 
Mistletoe  in  the  opposite  direction.  The  Mistletoe  lost  the  sound  of  the 
Putnam's  whistle  in  two  minutes.  The  Putnam  then  stopped  and  re- 
mained at  rest,  while  the  Mistletoe  continued  on  her  course  until  the 
Putnam  lost  sound  of  her  whistle,  twenty-six  minutes  later.  As  both 
steamers  were  separating  during  the  first  two -minutes  with  equal  speed, 


RESEARCHES    IN    SOUND.  525 

the  distance  travelled  by  the  sound  heard  moving  against  the  wind  is 
represented  by  4,  while  the  distance  of  the  sound  heard  with  the  wind 
is  represented  by  26  -f  2=28,  the  ratio  being  1:7.  It  should  be  men- 
tioned, however,  that  the  notes  in  this  experiment  are  defective  and 
somewhat  discrepant. 

Third  experiment. — The  Putnam  went  against  the  wind,  the  Mistletoe 
in  the  opposite  direction.  The  Putnam  lost  the  sound  of  the  whistle  of 
the  Mistletoe  in  two  minutes,  while  the  Mistletoe  continued  to  hear  the 
whistle  of  the  Putnam  ten  minutes  longer.  Owing  to  a  misunderstand- 
ing, one  of  the,  steamers  stopped  for  two  minutes  and  then  resumed  its 
course.  As  both  steamers  were  separating  during  the  first  two  minutes 
with  equal  speed,  the  distance  of  the  sound  heard  moving  against  the 
wind  is  represented  by  4,  while  the  sound  heard  with  the  wind  through 
a  space  denoted  by  2  x  10  -f  4—2=22,  the  ratio  being  1 :  5J. 

Fourth  experiment. — The  vessels  again  changed  directions,  the  Putnam 
going  with  the  wind,  and  the  Mistletoe  in  the  opposite  direction.  The 
Mistletoe  lost  the  sound  in  two  minutes,  and  the  Putnam  nine  minutes 
later.  As  each  steamer  was  moving  from  the  other  at  the  same  rate, 
the  distance  of  the  sound  heard  moving  agaiast  the  wind  would  be  rep- 
resented by  4,  while  the  distance  of  the  sound  moving  with  the  wind 
would  be  represented  b}^  9  X  2  +  4=22,  the  ratio  being  again  1 :  5J. 

Fifth  experiment. — This  experiment  was  made  August  10,  by  the  same 
vessels  and  same  observers,  wind  W.  S.  W.,  of  about  the  same  intensity 
as  on  previous  days ;  barometer,  30.1  ins. ;  dry  bulb,  74°  F.  wet  bulb,  69°. 
The  Putnam  steamed  against  the  wind,  and  the  Mistletoe  in  the  opposite 
direction.  The  Putnam  lost  the  sound  in  two  minutes,  and  the  Mistle- 
toe nine  minutes  later.  The  two  vessels  moving  apart  with  equal  veloc- 
ity, the  space  traversed  by  the  sound  moving  against  the  wind  was  rep- 
resented by  4,  while  that  in  the  opposite  direction  was  represented  by 
22,  viz,  9x2  +  4=22. 

Sixth  experiment. — The  vessels  were  next  separated  in  a  direction  at 
right  angles  to  the  wind,  when  each  reciprocally  lost  the  sound  of  the 
other  on  an  average  of  six  minutes,  giving  a  distance  travelled  by  the 
sound,  while  audible,  of  twelve  spaces. 

Seventh  experiment. — The  vessels  were  next  directed  along  an  interme- 
diate course  between  the  direction  of  the  wind  and  a  line  at  right  angles 
to  it,  with  the  following  results  :  The  Mistletoe,  against  the  wind,  lost 
the  sound  in  about  two  minutes,  while  the  Putnam  heard  the  sound 
seven  minutes  longer.  As  in  the  previous  case,  the  two  vessels  moving 
apart  with  equal  velocity  would  in  two  minutes  be  separated  by  a  space 
represented  by  4,  which  would  indicate  the  audibility  of  the  sound  mov- 
ing against  the  wind,  and  for  the  same  reason  the  other  vessel,  hearing 
the  sound  seven  minutes  longer,  would  have  the  additional  space  repre- 
sented by  14,  and  adding  to  this  four  spaces,  we  have  18  to  represent 
the  audibility  of  the  sound  in  the  direction  approximating  that  of  the 
wind. 


RESEARCHES    IN    SOUND. 


The  following  table  exhibits  at  one  view  the  results  of  the  foregoing 
experiments,  wJiieh  relate  to  sound  moving  against  tlxe  wind,  and  with 
the  wind,  reduced  to  miles. 


Experiment. 

Sound  with  the  wind. 

Sound  against  wind. 

1 

Miles. 
3.66 

Miles. 
066 

2     ^ 

4.66 

0.66 

3  ^  

3.66 

0.66 

4           .              „ 

3  66 

066 

5.    ^  

3.66 

0.66 

These  results  are  in  accordance  with  those  of  all  the  direct  observa- 
tions which  had  previously  been  made  on  sound  in  regard  to  wind  by 
the  Light-House  Board,  with  the  exception  of  those  at  Sandy  Hook  in 
September,  1874,  as  given  in  the  last  report,  in  which  the  sound  was 
heard  from  a  steamer  farther  against  the  wind  than  in  the  direction  of 
the  wind.  This  anomaly  was  explained  by  the  existence  of  an  upper 
currejit  of  air,  moving  in  an  opposite  direction  to  that  at  the  surface,  jn 
accordance  with  the  hypothesis  of  the  refraction  of  sound. 

It  will  be  observed  that  four  of  the  experiments  give  exactly  the  same 
distances  to  represent  the  audibility  of  sound  with  and  against  the 
wind.  This  coincidence  was  not  observed  until  after  the  notes  were 
collated  for  discussion,  and,  if  not  accidental,  was  due  to  the  equal  ve- 
locity of  the  wind  and  the  general  conditions  of  the  atmosphere  on  the 
two  days. 

To  give  a  definite  idea  of  these  relations  we  have  plotted  the  results 
obtained  on  August  10,  in  Fig.  5,  converting  the  distances  into  miles  and 
referring  them  to  a  common  center,  and  tracing  through  the  several  ex- 
tremities of  the  lines  representing  the  distances  a  continuous  line,  which 
may  be  designated  as  the  curve  of  audibility.  C  being  the  center  to 
which  the" sounds  are  referred,  O  A.  represents  the  distance  at  which  the 
sound  was  heard  against  the  wind,  and  CB  in  the  direction  of  the  wind, 
while  0  E  and  0  D  represent  the  distance  at  right  angles  to  the  wind? 
andj?  0  and  0  G-  the  distances  respectively  with  and  against  the  wind 
on  an  intermediate  course. 


RESEARCHES    IN    SOUND. 
FIG.  5. 


527 


The  curve  which  is  presented  in  the  foregoing  figure  may  be  considered 
as  that  which  represents  the  normal  limit  of  audibility  during  the  two 
days  in  which  the  experiments  were  made.  The  line  D  E  divides  the 
plane  of  the  <?urve  into  two  unequal  portions,  D  A  F  E.  and  D  G  B  E, 
the  former  representing  the  audibility  of  sound  moving  against  the 
wind,  and  the  other  the  audibility  of  sound  moving  with  the  wind. 

We  can  scarcely  think  that  any  other  condition  of  the  air  than  that 
of  its  motion  could  produce  a  result  of  this  kind.  It  exhibits  clearly  the 
fact  that  sound  is  not  heard  as  a  general  rule  at  right  angles  to  the  wind 
farther  than  with  the  wind,  as  has  been  asserted.  In  this  case  the  ratio 
of  the  latter  to  the  former  is  as  11  to  6,  or  nearly  double. 

The  investigation  of  the  relation  of  wind  to  the  penetration  of  sound 
was  renewed  in  a  series  of  subsequent  experiments,  the  results  of  which 
are  to  be  given  in  a  succeeding  part  of  this  report, 

It  should  be  observed,  in  comparing  Fig.  5  with  the  subsequent  fig- 
ures representing  the  curve  of  audibility,  that  the  arrow  representing 
the  direction  of  the  wind  points  in  the  longest  direction  to  the  figure, 
whereas  in  other  figures  the  pointing  is  in  the  opposite  direction.  The 
difference  arises  from  the  fact  that  in  Fig.  5  the  sound  is  supposed  to 
radiate  from  the  center,  C,  while  in  the  others  the  sound  converges  to  .the 
center  as  a  point  of  observation.  The  foregoing  diagram  and  all  that 
follow  in  this  report  were*  plotted  by  Mr.  Edward  Woodruff,  assistant 
superintendent  of  construction  of  the  third  Jight-house  district. 


528  EESEARCHES  IN  SOUND. 

EXPERIMENTS  AT  LITTLE  GULL  ISLAND,  SEPTEMBER,  1875. 

The  next  series  of  experiments  made  during  this  season  was  at  Little 
Gull  Island,  at  the  east  end  of  Lond  Island  Sound.  This  location  was 
chosen  on  account  of  its  convenience  of  approach  from  the  harbor  of 
Kew  London,  seven  miles  distant,  at  which  the  light-house  steamers  of 
the  third  district  usually  remain  when  not  engaged  in  active  service, 
and  also  because  there  is  a  light-house  on  the  island  furnished  with  two 
sirens  of  the  second  order,  and  an  extent  of  water  on  every  side  which 
would  allow  the  vessels  used  in  the  experiments  to  proceed  from  the 
island  as  a  center  to  a  considerable  distance  in  every  direction.  The 
island  itself  is  a  small  protuberance  above  the  water,  merely  sufficient 
in  area  to  support  a  raised  circular  platform  of  about  100  feet  in  diam- 
eter, on  which  the  light-house  and  other  buildings  are  erected.  The 
following  sketch  (Fig  6)  will  give  an  idea  of  the  position  of  Gull  Island 
relative  to  the  main-land  and  the  islands  in  the  vicinity. 

From  this  it  will  be  seen  that  the  position  was  not  the  most  favorable 
for  a  stable  condition  of  the  atmosphere.  As  the  heat  of  the  sun  in- 
creases during  the  first  part  of  the  day,  the  temperature  of  the  land 
rises  above  that  of  the  sea  ;  and  this  excess  of  temperature  produces 
upward  currents  of  air,  disturbing  the  general  flow  of  wind  both  at  the 
surface  of  the  sea  and  at  an  elevation  above.  But  although  the  locality 
was  unfavorable  for  obtaining  results  tending  to  exhibit  the  effects  of 
broad  currents  of  wind  flowing  in  one  direction,  it  had  the  advantage 
of  offering  more  varied  phenomena  than  could  otherwise  have  been  ex- 
hibited. Before  commencing  the  experiments,  directions  were  given  to 
attach  a  rotating  iron  neck  to  the  trumpet  of  one  of  the  sirens,  in  order 
that  it  might  be  directed  to  the  zenith  ,•  while  the  other  siren  remained 
with  its  axis  in  a  horizontal  direction.  The  observers  in  these  investi- 
gations consisted  of  the  chairman  of  the  board  5  General  Woodruff, 
engineer  third  district;  Porter  Barnard,  assistant  superintendent  of 
construction;  Captain  Keeney,  and  other  officers  of  the  Mistletoe;  with 
an  assistant  who  acted  as  one  of  the  observers  and  recording  clerk.  The 
Mistletoe  was  daily  employed,  though  on  two  occasions  the  Cactus, 
another  of  the  light-house  steamers,  rendered  assistance. 


RESEARCHES   IN   SOUND. 


529 


Fig.  6. 


J\/eir  LondorL     j 


BartletCs   Reef  Lt.V. 


Plum.  Id. 


Fisher  s  Id. 
Rctce  Rock 


jMtle  Gull  Id.  LH. 


Little-  Gull 
8?    Vicinity 

Scale  of  rrvil.es 


S.  Mis.  59 34 


530  RESEARCHES  IN  SOUND. 

OBSERVATIONS  ON  THE  ECHO  AT  LITTLE  GULL  ISLAND. 

The  first  observations  to  be  mentioned  are  those  relating  to  the  echo ; 
the  results,  however,  in  regard  to  this  are  not  very  satisfactory.  The 
sirens  were  of  the  second  order,  and  therefore  the  echoes  produced  were 
not  as  distinct  as  those  from  the  larger  instrument  at  Block  Island. 
The  echo  from  the  horizontal  trumpet  was  distinct,  and  in  the  prolonga- 
tion of  its  axis ;  the  interval  however  between  the  blast  of  the  siren 
trumpet  and  the  commencement  of  the  echo  was  very  brief;  so  short, 
indeed,  that  the  ending  of  the  one  and  the  beginning  of  the  other  were 
generally  difficult  to  distinguish.  A  slight  leak  in  the  apparatus  of  the 
siren  produced  a  continuous  hum,  which  interfered  somewhat  with  the 
distinct  appreciation  of  the  sound  of  the  echo.  The  keeper  thought  the 
weather  was  not  favorable  for  the  protiuction  of  echoes.  He  thinks 
they  are  heard  most  distinctly  during  a  perfect  calm,  which  did  not  occur 
during  the  course  of  these  investigations. 

The  axis  of  the  siren  with  the  movable  trumpet  being  directed  to  the 
zenith,  strict  attention  was  given  by  all  the  observers  to  any  echo  which 
might  be  produced  from  it;  but  in  this  case, as  in  that  at  Block  Island, 
the  slight  echo  which  was  heard  came  from  all  points  of  the  horizon. 
On  one  occasion  General  Woodruff  called  attention  to  a  small  cloud  pass- 
ing directly  over  the  zenith,  from  which  a  few  drops  of  rain  fell  upon  the 
platform  on  which  the  light-house  is  erected.  Advantage  was  taken  of 
this  occurrence  to  direct  strong  blasts  of  the  siren  toward  the  cloud, 
but  no  perceptible  echo  was  returned.  We  have  failed,  therefore,  in 
this  series  of  investigations,  to  obtain  any  positive  facts  in  addition  to 
those  already  known  as  to  the  character  of  the  echo.  In  regard  to  the 
hypothesis  offered  for  its  explanation,  if  we  found  little  in  its  support, 
we  have  met  with  nothing  to  invalidate  it.  But  whatever  may  be  the 
cause  of  the  phenomenon,  we  do  not  consider  it  an  important  factor  in 
explanation  of  the  results  we  have  obtained,  since  it  was  too  feeble  to 
produce  any  effect  in  the  way  of  absorbing  any  notable  part  of  the 
original  sound.  Its  importance  from  Dr.  Tyndall's  point  of  view  is  its 
apparent  support  of  the  hypothesis  of  a  flocculent  condition  of  the 
atmosphere. 

OBSERVATIONS  ON  EFFECT  OF  ELEVATION  ON  AUDIBILITY. 

The  next  class  of  experiments  at  Little  Gull  Island  had  relation  to 
the  effect  of  elevation  on  sound.  The  conditions  here,  however,  for  ar- 
riving at  definite  results  on  this  point  were  by  no  means  as  favorable  as 
those  at  Block  Island.  The  height  which  could  be  commanded  was  only 
that  of  the  tower  of  the  light  house,  the  gallery  of  which  is  74  feet 
above  the  platform  upon  which  the  buildings  are  erected,  and  92  feet 
above  the  level  of  the  sea,  much  less  than  that  at  Block  Island.  Be- 
sides this,  the  variableness  of  the  wind  at  the  surface  of  the  ocean  and 
at  heights  above  was  not  favorable  for  the  illustration  of  the  point  in 
question. 


RESEARCHES    IN    SOUND. 


531 


The  theoretical  conditions  in  order  that  the  sound  may  be  heard  with 
greater  distinctness  at  an  elevation  than  below  are,  as  we  have  said 
feefore,  that  the  wind  be  moving  with  a  greater  velocity  in  a  given  di- 
rection at  an  elevation  than  at  the  surface  of  the  earth,  and  that  the 
difference  in  the  velocities  may  be  against  the  sound-wave,  so  that  its 
upper  part  may  be  more  retarded  than  the  lower.  In  this  case  the  di- 
rection of  a  beam  of  sound  will  be  curved  upward,  leaving  as  it  were 
a  va-cuum  of  sound  beneath.  The  distance  of  the  origin  of  sound,  how- 
ever, must  not  be  too  great  relatively  to  the  elevation  of  the  observer; 
otherwise  it  will  pass  over  his  head,  as  well  as  over  that  of  the  observer 
at  the  surface  of  the  earth.  In  most  instances  the  sound  was  not  con- 
tinuous, but  was  interrupted — heard  for  a  time,  then  lost;  again  be- 
coming audible,  it  was  heard  until  it  finally  became  imperceptible. 
Besides  this,  it  was  difficult  to  determine  when  the  sound  ceased  to  be 
heard,  since  this  depended  on  the  sensibility  of  the  ear  and  the  greater 
or  less  attention  of  the  observer  at  the  time  of  the  observation.  To 
obviate  these  difficulties  as  well  as  the  unfavorable  condition  of  too 
great  a  distance  of  the  origin  of  sound  from  the  observer,  it  was  con- 
cluded to  adopt  as  the  duration  of  the  sound  the  elapsed  time  between 
its  beginning  and  the  period  when  it  was  first  lost. 

The  observer  on  the  tower  was  Mr.  P.  Barnard,  while  the  one  below 
was  General  Woodruff.  From  the  records  of  the  observations  of  these 
gentlemen  the  following  tables  are  compiled,  the  first  of  which  indicates 
the  relative  duration  of  sound  on  the  top  of  the  tower  and  at  the  bottom, 
the  sound  moving  against  the  wind;  the  second,  the  same  duration,  the 
sound  moving  with  the  wind;  and  the  third,  the  same  with  the  sound 
at  right  angles  to  it. 

TABLE  1. — Sound  against  tlie  wind. 


Date. 

Heard  at 
top  of 
tower. 

Heard  at 
bottom. 

1875. 
September  2  

min.  sec. 
5    30 

min.  sec. 
4    00 

4  .                 

4    30 

3    30 

4  

5    30 

3    00 

4  .... 

5    00 

4    00 

6  

7    00 

2    15 

6  

4    00 

3    00 

7  

5    00 

2    15 

8  

6    00 

4    00 

8  

5    30 

3    45 

8  

3    30 

2    15 

8          ,  

3    00 

1    15 

Mean  ,         

4    57 

3    01 

532 


RESEARCHES   IN   SOUND. 


It  appears  from  Table  1  that  without  a  single  exception  the  duration  of 
the  sound  was  greater  at  the  top  of  the  tower  than  at  the  bottom,  al- 
though the  difference  in  favor  of  the  top  of  the  tower  in  the  several  experi- 
ments is  very  variable.  These  results  are  in  accordance  with  what  was 

anticipated. 

TABLE  2. — Sound  with  the  wind. 


Date. 

Heard  at 
top   of 
tower. 

Heard  at 
foot  of 
tower. 

September  2  .  .        .         

min.  sec. 
30    00 

min.  sec. 
30    00 

3  

16    30 

18    00 

4  

2i     00 

20    30 

4  

18    00 

23    30 

6  

12    30 

12    30 

7  

6    30 

5    30  ' 

Mean                     .       .              .                

17    25 

18    20 

In  these  observations  the  duration  of  the  sound  at  the  bottom  and  top 
are  nearly  the  same,  from  which  we  might  infer  that  the  elevation  of 
the  observer  has  little  effect  on  the  hearing  of  sound  moving  with  the 
wind.  Were  it  not  for  the  result  of  the  first  experiment  of  this  class  at 
Block  Island,  we  should  not  hesitate  to  adopt  this  as  a  general  conclu- 
sion. 

TABLE  3 — Sound  heard  nearly  at  right  angles  to  the  wind. 


Date. 

Heard  at 
top   of 
tower. 

Heard  at 
foot  of 
tower. 

September  2...     <                 .. 

min.  sec. 
6    00 

min.  sec. 
4    00 

2           .     .         ..           

6    45 

10    00 

2    ,  

25    00 

23    00 

2                                       ....... 

16    30 

4    00 

3                   

21    00 

19    15 

3    

16    00 

14    30 

3  

23    30 

16    45 

4          .                          .                   

19    30 

17    30 

6             

6    30 

5    30 

7  

5    00 

6    45 

7    .                                  

12    00 

12    30 

8                   

4    15 

3    15 

8                                           .                    

9    30 

5    00 

Mean       

13    12 

10    55 

RESEARCHES    IN    SOUND. 


533 


From  the  result  of  this  table  it  would  appear  that  the  sound  can  be 
heard  moving  at  right  angles  to  the  wind  better  at  an  elevation  that  at 
the  surface  —  a  result  not  anticipated. 

OBSERVATIONS  ON  THE  EFFECT  OF  WIND  ON  SOUND. 

This  series  was  commenced  on  the  2d  of  September.  Barometer,  30.3 
inches  ;  thermometer,  dry-bulb,  70°.5  F.  wet-bulb,  67°.5.  Wind  at  the 
surface  of  sea  was  six  miles  per  hour,  and  variable  ;  at  3  p.  m.  the  ve- 
locity was  eight  miles  at  the  surface.  (See  Fig.  7.) 


Fig.  7. 


1sl  course 


The  experiments  were  made  by  means  of  the  steamer  Mistletoe,  which 
proceeded  from  the  light-house,  as  a  center,  in  different  directions, 
blowing  the  whistle  every  half-minute,  and  returning  when,  from  a  sig- 
nal, the  sound  was  lost ;  the  time  being  noted  by  different  observers, 
and  the  distance  estimated  by  the  position  of  the  steamer  in  reference 
to  known  objects  on  the  Coast- Survey  chart,  as  well  as  by  angles  of 
azimuth  and  time  of  sailing.  The  steamer  was  directed  to  proceed,  as 
indicated  in  Fig.  7,  1st,  against  the  wind,  so  that  the  sound  would  come 
to  the  observers  with  the  wind ;  2d,  at  right  angles  to  the  wind ;  3d,  in 
an  intermediate  direction  between  the  last  course  and  the  direction  of 
the  wind ;  4th,  approximately  with  the  wind,  so  that  the  sound  would 
come  to  the  ears  of  the  observers  against  the  wind ;  5th,  in  an  interme- 
diate direction  j  and,  6th,  again  at  right  angles  to  the  wind.  It  was 


534  RESEARCHES   IN   SOUND. 

supposed  that  by  this  arrangement  a  symmetrical  curve  of  sound  would 
be  obtained ;  and  we  think  this  would  have  been  the  case  had  the  wind 
remained  constant  in  direction.  It  did  remain  nearly  the  same  during 
the  time  of  describing  the  first,  second,  and  third  courses,  and  only 
slightly  varied  during  the  fourth ;  but  previous  to  running  the  fifth  and 
sixth  courses  the  wind  had  changed  to  a  direction  nearly  at  right  angles 
to  its  first  course. 

As  is  shown  in  Fig.  7,  the  first,  second,  third,  and  fourth  courses  form 
a  normal  curve  of  audition ;  the  fifth  and  sixth  courses,  however,  give 
discordant  results,  being  much  longer  than  a  symmetrical  curve  would 
indicate,  showing  a  change  in  the  condition  of  the  medium  from  that 
which  existed  during  the  running  of  the  other  courses ;  this  change  was 
evidently  that  of  the  wind,  which,  veering,  as  we  have  seen,  through 
an  arc  of  a  little  more  than  90°,  brought  it  nearly  at  right  angles  to  the 
fifth  course,  and  approximately  in  the  direction  of  the  sixth  course ;  the 
wind  also  increased  its  velocity.  These  changes  are  sufficient,  without 
other  considerations,  to  give  a  rational  account  of  the  phenomena  ob- 
served. They  both  tend  to  increase  the  distance  at  which  the  sound 
would  be  heard. 

In  these  experiments,  as  in  subsequent  ones,  it  is  to  be  regretted  that 
for  want  of  balloons  the  motion  of  the  air  above  could  not  be  ascer. 
tained,  as  was  done  at  Sandy  Hook  in  September,  1874.  Previous  to 
sailing  from  the  depot  at  Staten  Island  attempts  had  been  made  to  se- 
cure a  supply  of  toy  balloons,  but  none  could  be  found  at  that  time  in 
the  city  of  New  York.  Arrangements  were  therefore  made  for  procur- 
ing a  reservoir  of  condensed  hydrogen,  by  which  India-rubber  balloons 
could  be  inflated  at  the  time  they  were  wanted.  Unfortunately  this  ap- 
paratus did  not  arrive  in  time  to  be  of  much  avail  in  this  series  of  experi- 
ments. Besides  this,  on  account  of  the  smallness  of  the  balloons,  the 
ascent  was  too  slow  compared  to  the  horizontal  motion  to  indicate  the 
direction  of  the  wind  at  a  considerable  elevation  above  the  points  of 
observation.  They  were  however  of  use  in  pointing  out  definitely  the 
direction  of  the  wind  and  the  changes  it  was  undergoing.  Moreover,  at 
the  time  of  leaving  New  York  we  were  only  able  to  procure  one  ane- 
mometer, whereas  we  ought  to  have  had  a  number,  one  for  the  top  of 
the  tower,  one  for  the  bottom,  and  one  for  each  vessel. 

Experiments  of  September  3. — Barometer,  30.02  inches ;  thermometer, 
dry  bulb,  72°.5  F.  wet  bulb,  70° ;  wind  from  the  east,  but  too  slight  to 
move  the  cups  of  the  anemometer ;  it  soon  however  spang  up  from  the 
opposite  direction,  in  which  it  continued  during  the  remainder  of  the 
day,  attaining  a  velocity  of  five  and  a  quarter  miles  per  hour. 

In  these  experiments  two  light-house  steamers  were  employed,  the 
Mistletoe  and  Cactus,  which  enabled  us  to  obtain  the  results  in  half  the 
time,  and  thus  to  obviate  the  effect,  in  some  degree,  of  any  change  in 
the  direction  of  the  wind.  On  this  occasion  the  sound  was  noted  at  the 
light-house  as  it  converged  to  a  center  from  the  whistle  of  each  vessel, 


RESEARCHES    IN   SOUND. 
Fig.  *. 


535 


536  RESEARCHES   IN   SOUND. 

and  also  simultaneously  by  each  vessel  as  it  diverged  from  the  vertical 
siren. 

We  were  enabled,  in  this  way,  to  produce  two  curves  by  a  reverse 
process.  These  are  plotted  in  Fig.  8,  and  exhibit  a  remarkable  degree 
of  similarity.  The  corresponding  parts  of  the  two  curves,  being  in  each 
case  reversed,  exhibit  the  fact  that,  through  the  same  space  in  opposite 
directions,  the  audibility  of  the  sound  was  similarly  increased  with  the 
wind  and  diminished  against  it.  The  effect  however  of  the  wind  in  the 
experiments  of  this  day  was  less  marked  than  on  any  in  the  whole 
series,  and  consequently  the  two  curves  of  audition  more  nearly  ap- 
proximate circles. 

We  can  see  in  this  result  no  other  effect  than  that  which  would  be 
produced  from  a  wind  flowing  with  a  uniform  but  slow  velocity  at  the 
surface,  but  having  a  slightly  increased  velocity  above.  Had  there  been 
no  wind,  according  to  this  view  the  two  curves  would  have  exhibited 
two  concentric  circles. 

Experiments  of  September  4. — Barometer,  29.85  inches,  falling ;  ther- 
mometer, dry  bulb,  77°  F.  wet  bulb,  73°.25.  Wind  south  by  west, 
twelve  and  one-fourth  miles  per  hour  at  the  top  of  the  tower  and  nine 
and  one-fourth  miles  below ;  variable. 

These  experiments  were  also  made  with  two  vessels.  The  distances 
and  directions  are  given  in  Fig.  9.  With  the  exception  of  the  fourth 
course  of  the  Cactus,  the  other  courses  would  form  nearly  a  symmetri- 
cal curve,  but  in  this  case  the  sound  of  the  whistle  of  the  Cactus  was 
lost  at  the  point  a  at  a  distance  of  one  mile,  and  was  afterward  regained 
at  the  point  &,  and  continued  audible  until  the  steamer  reached  the 
point  c. 

This  presents  one  of  the  abnormal  phenomena  of  sound  which  might 
in  part  be  accounted  for  by  the  existence  of  a  flocculent  cloud  between 
a  and  &,  but  why  the  sound  could  be  heard  so  much  farther  in  this  direc- 
tion than  in  the  others  is  not  easy  to  explain  on  that  hypothesis. 

The  line  I  c  was  described  after  all  the  lines  of  Fig.  9  had  been  com- 
pleted, and  therefore  the  curve  given  in  the  figure  correctly  represents 
the  boundary  of  the  area  of  audition  while  these  courses  were  being  run, 
the  point  a  being  the  termination  under  that  condition  of  the  fourth 
course  of  the  Cactus.  To  explain  the  abnormal  line  b  c,  we  have  only  to 
suppose  that  a  change  in  the  velocity  of  the  wind  afterward  took  place, 
by  which  its  opposition  to  the  sound-wave  was  diminished ;  this  will  ac- 
count for  the  greater  length  of  the  line ;  the  change  however  did  not 
reach  the  light-house  until  after  the  vessel  had  passed  the  point  &. 


RESEARCHES   IN -SOUND. 

Fig.  9. 

Cactus 
^t-7^ course 


537 


As  affording  evidence  in  support  of  this  hypothesis,  it  may  be  men- 
tioned that  on  examining  the  records  of  the  Signal-Service,  of  which  there 
is  a  station  at  New  London,  seven  miles  north  of  the  position  at  which 
these  observations  were  made,  it  was  found  that  the  wind  in  the  morn- 
ing of  that  day  was  south,  in  the  afternoon  southwest,  and  in  the  even- 
ing northwest,  and  that  it  was  probable,  as  in  other  cases,  that  the  wind 
had  changed  above  while  the  part  of  the  course  &  c  was  run. 

Experiments  of  September  6. — Barometer,  29.93  inches ;  thermometer, 
dry  bulb,  74°.5  F.  wet  bulb,  67° ;  wind  from  northwest  to  southwest, 
seventeen  miles  per  hour.  The  wind,  though  of  higher  velocity  than  on 
any  other  occasion,  was  variable.  On  this  day  the  experiments  were 
principally  made  with  the  Mistletoe.  The  Cactus,  being  obliged  to  leave 
on  other  duty,  ran  one  course  a  distance  of  two-thirds  of  a  mile  before 
the  sound  of  her  whistle  was  lost  at  the  light-house.  She  afterwards 
steamed  off  in  the  direction  C  &  (Fig.  10),  noting  the  sound  of  the  siren, 
which  was  lost  at  the  point  &,  afterward  regained,  and  heard  distinctly 
ten  and  one-half  miles  distant. 

During  the  passage  of  the  first  course  of  the  Mistletoe,  the  wind  at 
the  surface  and  above  was  from  southwest,  the  latter  being  indicated 


538 


RESEARCHES    IN    SOUND. 


by  a  cloud  passing  the  zenith.  During  the  second  course  the  wind  was 
variable,  changing  its  direction  about  90°,  principally  from  the  north- 
west ;  while  during  the  third  course  the  wind  was  again  from  the  south- 
west. The  long  course  of  the  Cactus  marked  on  the  figure  indicates  the 
sound  of  the  siren,  from  the  center  outward,  as  it  was  heard  seven  and 
one-fourth  miles,  then  lost  for  an  interval,  and  afterward  heard  again  at 
a  distance  of  three  and  one-fourth  miles  farther,  making,  in  all,  ten  and 

one-half  miles. 

Fig.  10. 


Experiments  of  September  7. — Barometer,  30.1  inches ;  thermometer, 
dry-bulb,  73°  F.  wet-bulb,  62° ;  wind,  eight  miles  per  hour  above,  and 
five  miles  per  hour  below,  tower.  The  wind  was  variable,  as  indicated 
by  the  letting-off  of  balloons,  which  however  did  not  rise  to  any  great 
height.  The  direction  of  the  wind  is  shown  in  Fig.  11  by  arrows.  There 
is  nothing  remarkable  in  the  curve  of  audition  of  this  day.  It  indicates, 
as  usual,  a  greater  distance  toward  the  side  on  which  the  sound  was 
moving  with  the  wind. 

Experiments  of  September  8. — Barometer,  30.3  inches  $  thermometer, 
dry -bulb,  71°  F.  wet-bulb,  64°.5;  wind,  west-southwest,  fifteen  miles 
per  hour  at' top  of  tower,  nine  miles  per  hour  below.  Fig.  12  indicates 
the  curve  of  audition  of  the  vertical  siren  as  compared  with  that  of  the 


EESEAECHES   IN   SOUND. 
Fig.  11. 


539 


horizontal  siren.  The  steamer  first  proceeded  along  the  line  C  a  nearly 
in  the  direction  of  the  axis  of  the  horizontal  trumpet.  For  the  distance 
of  the  first  three  miles  the  horizontal  trumpet  was  the  louder.  At  the 

Fig.  12. 


point  a,  four  miles  distant,  the  two  were  distinct  and  very  nearly  equal. 
At  b  they  were  distinct,  also  very  nearly  equal,  the  vertical  perhaps  a 
little  more  distinct.  At  c  very  nearly  equally  distinct.  At  d  the  verti- 
cal siren  was  decidedly  more  distinct  just  before  entering  the  optical 


540  RESEARCHES   IN   SOUND. 

shadow  of  the  light-house  tower  and  the  keeper's  dwelling.  This  shadow 
continued  to  the  point  e,  which  was  nearly  the  extent  of  the  acoustic  as 
well  as  of  the  optical  shadow,  since  from  d  to  e  the  sound  was  heard 
from  neither  instrument,  and  the  origin  of  sound  was  too  near  to  cause 
much  diiference  between  these  two  shadows.  From/  to  a,  through  the 
point  </,  the  two  instruments  continued  to  be  fully  heard — the  vertical 
the  more  distinct.  The  effect  of  the  wind  in  this  figure  is  also  very 
distinctly  marked,  the  longer  lines  indicating  the  distance  the  sound 
was  heard  with  the  wind,  and  the  shorter  against  it.  The  curve  of  this 
figure  is  not  traced  through  points  at  which  the  sound  was  absolutely 
lost,  but  at  which  it  was  heard  feebly  and  with  nearly  equal  distinctness. 

Thus  far  all  the  facts  we  have  observed,  if  not  in  strict  conformity 
with  our  conception  of  the  hypothesis  of  Professor  Stokes,  are  at  least 
not  incompatible  with  it.  We  are  now  however  to  direct  attention  to 
a  fact  of  much  interest,  which  may  not  have  escaped  the  attention  of 
the  reader ;  namely,  the  remarkable  difference  in  the  area  of  audition 
as  exhibited  in  the  several  figures,  all  drawn  to  the  same  scale.  Com- 
pare, for  example,  the  curve  of  Fig.  10  with  the  inner  curve  of  Fig.  8. 
It  might  at  first  sight  be  inferred  that  the  srnallness  of  the  curve  in 
the  former  case  was  due  to  a  mottled  condition  of  the  atmosphere, 
which,  by  absorbing  the  sound,  diminishes  the  sphere  of  audition  ;  but, 
unfortunately  for  this  explanation,  it  would  appear  from  the  observa- 
tions made  by  the  Cactus  within  the  hour  of  obtaining  the  data  for 
describing  the  curve,  that  the  air  was  then  in  a  remarkably  favorable 
condition  for  the  transmission  of  sound,  since  it  was  heard  ten  and  a 
half  miles,  the  ordinary  limit  of  the  maximum  penetrating  power  of  the 
instrument — a  siren  of  the  second  order;  while  on  the  3d  of  September, 
the  day  on  which  the  large  curve,  Fig.  8,  was  described,  the  greatest 
distance  at  which  the  sound  of  the  same  instrument  could  be  heard  was 
eight  and  a  quarter  miles. 

The  only  difference  in  the  condition  of  the  air  observed  during  the 
time  of  describing  the  curve  of  audition  given  in  the  figure,  and  the 
hearing  of  the  sound  by  the  Cactus  for  ten  and  a  half  miles,  was  a 
change  in  the  direction,  and  perhaps  in  the  intensity,  of  the  wind,  in  the 
latter  case  the  direction  being  the  same  as  that  of  the  course  of  the 
Cactus. 

Before  therefore  admitting  any  other  solution  of  the  question  as  to 
the  cause  of  the  difference  in  the  area  of  audition,  we  must  inquire 
whether  it  is  not  possible  to  refer  it  to  the  action  of  the  wind  itself. 

The  most  marked  difference  in  the  conditions  which  apparently  af- 
fected the  phenomenon  on  the  days  in  question  was  that  of  the  greater 
velocity  of  the  wind,  both  at  the  surface  of  the  sea  and  at  the  top  of  the 
tower,  and  by  comparing  the  several  figures  in  regard  to  the  wind  it 
will  be  seen  that  where  the  condition  of  the  air  was  nearest  that  of  a 
calm  the  larger  was  the  curve  of  audition,  and  the  nearer  the  figures 
approach  to  a  circle,  of  which  the  point  of  origin  of  sound  or  the  point 


EESEARCHES   IN   SOUND.  541 

of  perception  is  the  center.  From  these  facts  we  are  inclined  to  think 
that  sound  is  not  heard  as  far  during  a  time  of  high  wind  in  any  direc- 
tion as  it  is  during  a  perfect  calm,  and  that  it  is  heard  farthest  with  a 
gentle  wind.  This  conclusion,  which  was  not  anticipated  at  the  begin- 
ning of  these  investigations,  is  we  think  in  strict  conformity  with  the 
hypothesis  adopted.  In  the  case  of  sound  moving  against  a  strong 
wind,  the  sonorous  waves  being  thrown  up  above  the  ears  of  the  ob- 
server, the  sphere  of  audition  in  that  direction  is  without  question 
greatly  diminished ;  and  that  it  should  be  also  diminished  when  sound 
is  moving  with  a  strong  wind  having  a  greater  velocity  above  than  be- 
low is  not  difficult  to  conceive.  In  this  case  the  sound-wave  will  be  so 
thrown  down  against  the  earth,  and  so  much  of  it  absorbed,  as  to  weaken 
the  intensity  of  that  part  which  reaches  the  ear,  while  in  the  case  of  a 
feeble  wind,  moving  faster  above  than  below,  the  portion  of  the  wave 
thrown  down  from  above  will  only  be  sufficient  to  compensate  for  the 
smaller  loss  by  friction,  and  thus  the  sound  may  be  heard  at  a  greater 
distance  than  in  still  air.  But  on  this  point,  as  well  as  others,  further 
experiments  are  required. 

While  we  consider  the  wind  as  the  principal  agent  in  producing  the 
abnormal  phenomena  of  sound,  we  do  not  by  any  means  regard  it  as 
the  sole  agent.  Prof.  Osborn  Reynolds,  of  Owens  College,  Manchester, 
without  any  knowledge  of  what  was  doing  in  America  on  this  subject, 
instituted  a  series  of  experiments  on  the  effect  of  wind  upon  sound,  and 
finally  adopted  precisely  the  same  hypothesis  which  we  have  used  for 
generalizing  the  observed  phenomena.  He  has  however  in  a  very  in- 
genious and  important  paper,  presented  to  the  Royal  Society  in  1874, 
extended  the  same  principle  to  the  effect  of  heat  in  changing  the  form 
of  the  sound-wave,  and  has  shown,  both  by  reasoning  and  experiment, 
that  the  normal  direction  of  the  sound-wave  in  still  air,  instead  of  pro- 
ceeding horizontally  should  be  turned  upward,  on  account  of  the  greater 
velocity  of  sound  near  the  earth,  due  to  the  greater  heat  of  the  strata  in 
that  position  than  of  those  above.  This  principle,  which  indicates  the 
existence  of  a  true  refraction  of  sound  independent  of  the  motion  of 
the  medium,  is  undoubtedly  applicable  as  a  modifying  influence  to  the 
phenomena  we  have  recorded.  It  produces  however  only  a  slight  ef- 
fect in  the  case  we  have  last  mentioned,  since  the  observation  on  board 
the  Cactus  shows  the  condition  of  the  air  was  that  of  little  acoustic  ab- 
sorption. It  would  nevertheless  favor  the  hypothesis  that  sound  in 
perfectly  still  air  of  homogeneous  density  could  be  heard  farther  than 
sound  in  a  moving  medium,  or  in  one  of  unequal  temperature.  This  is 
also  in  accordance  with  the  fact  repeatedly  observed  in  arctic  regions, 
in  which  the  sound  of  the  human  voice  is  heard  at  great  distances  dur- 
ing times  of  extreme  cold.  In  this  case,  the  air  is  of  a  uniform  temper- 
ature above  and  below,  but  of  diminished  elasticity,  and  should,  on  this 
account,  transmit  sound  with  less  intensity ;  and  yet  the  audibility  is 
increased,  which  is  explained  by  the  assumption  that  its  stillness  and 


542  RESEARCHES   IN   SOUND. 

uniformity  of  temperature  more  than  compensate  for  the  diminished 
elasticity.  The  same  may  be  said  with  regard  to  the  audibility  of  sound 
during  a  fog,  which  usually  exists  during  extreme  stillness  of  the  air. 

Whatever  be  the  cause  of  the  variation  in  the  limit  of  audition  as 
exhibited  in  the  diagrams,  it  is  less  efficient  than  the  ordinary  action  of 
the  wind  in  producing  the  same  phenomena.  This  is  evident  from  the 
fact  that  while  the  ratio  of  the  extreme  variation  in  the  limits  of  audi- 
tion in  the  first  case  is  not  more_  than  1:3,  in  the  second  it  is  that  of 
1:5. 

Moreover,  when  the  effect  of  the  wind  on  the  audition  of  sound  in 
relation  to  elevation  is  considered,  we  think  we  are  fully  warranted  in 
asserting,  as  we  did  in  our  last  report,  that  the  wind  is  a  more  efficient 
cause  of  the  variability  of  the  penetration  of  sound  than  the  invisible 
acoustic  clouds  adopted  by  Professor  Tyndall  for  the  explanation  of  the 
phenomena. 

The  object  of  these  investigations,  as  stated  at  the  beginning  of  this 
report,  was  to  obtain  facts  which  might  serve  to  establish  the  true  theory 
of  the  abnormal  phenomena  of  sound,  an  object,  independent  of  its 
scientific  interest,  of  much  practical  importance  in  its  application  to  fog- 
signals.  Although  the  observations  were  not  as  perfect  as  we  could 
wish  in  many  respects,  from  want  of  certain  appliances,  they  are  yet 
sufficient  we  think  to  establish  principles  of  much  practical  value.  For 
example,  if  the  mariner  in  approaching  a  fog-signal  while  the  wind  is 
blowing  against  the  sound  fails  to  perceive  it  on  deck,  he  will  probably 
hear  it  by  ascending  to  the  mast-head ;  or,  in  case  a  sound  from  a  given 
station  is  constantly  obscured  in  a  certain  direction,  while  it  is  audible 
in  adjacent  directions,  we  may  attribute  it  to  a  sound-shadow  produced 
by  some  interposing  object.  If  again,  the  obscuration  of  sound  in  a 
given  direction  is  only  observed  during  a  wind  moving  against  the  sound, 
the  cause  will  probably  be  found  in  a  lateral  refraction,  due  to  the  re- 
tardation of  the  current  of  wind  against  a  perpendicular  wall  or  cliff,  as 
in  the  3ase  observed  at  Block  Island,  August  19.  The  subject  however 
is  one  of  great  complexity,  and  requires  further  investigation,  but  the 
results  thus  far  obtained  may  be  considered  as  furnishing  the  preliminary 
data  on  which  to  found  more  precise  observations.  These  should  be 
made  with  the  aid  of  a  number  of  steamers  simultaneously  employed, 
each  furnished  with  anemometers  and  balloons  for  determining  with  more 
accuracy  the  direction  and  velocity  of  the  wind. 

We  hope  to  renew  the  investigations  during  next  summer,  and  in  view 
of  this  have  directed  that  in  the  mean  time  the  light-keepers  at  Block 
Island  and  at  Point  Judith  shall  continue  to  sound  their  sirens  a  certain 
length  of  time  every  Monday,  noting  the  direction  and  velocity  of  the 
wind,  the  temperature  and  pressure  of  the  air,  and  the  audibility  of  the 
sound  as  it  comes  reciprocally  from  each  instrument. 

It  is  shown,  from  the  results  thus  far  obtained  from  these  reciprocal 
observations,  that  sound  is  occasionally  heard  more  distinctly  against 


RESEARCHES   IN   SOUND.  543 

the  wind  than  in  a  contrary  direction.    We  think  however  that  these 
instances  are  generally  followed  by  a  change  in  the  direction  of  the  wind 
at  the  surface  of  the  earth. 
LIGHT-HOUSE  BOARD,  October,  1875. 


PAET  V.— INVESTIGATIONS  IN  1877.* 

On  account  of  the  occurrence  of  the  Centennial  Exhibition,  which 
absorbed  most  of  the  time  of  the  officers  of  the  Light-House  Board  not 
devoted  to  ordinary  light-house  service,  but  few  observations  were  made 
relative  to  sound  in  1876,  and  an  account  of  what  were  made  is  incor- 
porated in  the  following  report. 

Agreeably  to  previous  engagement,  I  visited  Portland,  Me.,  to  make 
SDine  investigation  in  regard  to  an  abnormal  phenomenon  of  sound, 
which  was  noticed  in  a  former  report.  We  left  Portland  on  the  after- 
noon of  September  3, 1877,  in  the  steamer  Iris,  which  had  been  fitted  up 
during  the  year  under  the  direction  of  the  inspector,  Commander  H.  F. 
Picking,  and  was  in  excellent  condition,  and  well  adapted  to  the  duty 
of  a  light-house  tender.  The  party  consisted  of  General  J.  C.  Duane, 
engineer  of  the  first  district ;  Commander  H.  F.  Picking,  inspector  of 
the  first  district ;  Mr.  Edward  L.  Woodruff,  assistant  engineer  of  the 
third  district ;  Mr.  Charles  Edwards,  assistant  engineer  of  the  first  dis- 
trict, and  myself. 

We  first  examined  one  of  the  automatic  whistling-buoys  invented  by 
Mr.  Courtenay,  of  New  York.  This  was  in  place  and  emitting  sounds 
at  a  station  called  Old  Anthony,  off  Cape  Elizabeth,  about  nine  miles 
from  Portland.  On  approaching  it  at  right  angles  to  the  direction  of 
the  wind,  we  heard  it  at  the  distance  of  a  mile.  But  the  sound  did  not 
appear  loud  even  within  a  few  rods.  It  was  however  of  considerable 
quantity,  being  from  a  locomotive  whistle  of  ten  inches  in  diameter. 
The  instrument  is  operated  by  the  oscillation  of  the  waves,  which  at 
this  time  were  not  of  sufficient  height  to  move  it  vertically  through  a 
space  of  more  than  one  foot.  It  emitted  a  sound  at  each  oscillation. 
This  invention  consists  of  a  large  pear-shaped  buoy  about  twelve  feet  in 
diameter  at  the  water-surface,  and  floats  about  twelve  feet  above  the 
same  plane.  In  the  interior  of  this  buoy  is  a  large  tube  or  hollow  cyl- 
inder, three  feet  in  diameter,  extending  from  the  top  through  the  bottom 
to  a  depth  of  about  thirty  feet  below  the  latter.  This  tube  is  open  at 
the  bottom,  but  projects  air-tight  through  the  upper  part  of  the  buoy, 
and  is  closed  with  a  plate  having  three  orifices  in  it,  two  for  letting  in 
the  air  into  the  tube,  and  one  between  the  others  for  letting  it  out  to 
operate  the  whistle.  These  orifices  are  connected  with  three  tubes 
which  extend  downward  to  near  the  level  of  the  water,  where  they  pass 
through  a  diaphragm  which  divides  the  cylinder  into  two  parts. 

*  From  the  Report  of  tlie  Light-House  Board  for  1877. 


544  RESEARCHES   IN   SOUND. 

When  the  buoy  rises,  the  water  in  the  cylinder  by  its  inertia  retains 
its  position,  and  a  partial  vacuum  is  formed  between  the  head  of  the 
column  and  the  diaphragm,  into  which  the  air  is  drawn  through  two  of 
the  tubes,  and  when  the  buoy  descends,  the  escape  through  the  injec- 
tion-tube being  prevented  by  valves,  it  is  forced  out  of  the  inner  tube 
and  actuates  the  whistle. 

The  mooring-chain,  which  is  sixty  fathoms  in  length,  is  attached  to 
the  cylinder  at  a  point  just  below  the  buoy,  and  is  secured  to  a  large 
stone  weighing  about  six  tons.  The  apparatus  rides  perpendicularly. 

The  sound  in  this  instrument  is  not  produced  merely  by  the  difference 
in  hydrostatic  pressure  of  the  water  in  the  two  positions  of  the  buoy, 
but  by  the  accumulated  effect  of  impulse  generated  by  the  motion  of 
the  apparatus. 

Plans  have  been  devised,  but  have  not  yet  been  perfected,  to  condense 
the  air  in  the  buoy  by  the  effect  of  repeated  oscillations,  until  a  valve 
loaded  to  a  definite  pressure  would  open  automatically  and  allow  the  air 
to  escape.  In  this  way  the  sound  from  the  accumulated  pressure  would 
be  produced  at  intervals  to  a  greater  or  less  extent,  and  would  serve  to 
diversify  the  character  of  the  sound  so  as  to  enable  the  mariner  to  dis- 
tinguish different  locations.  The  invention,  as  it  is,  is  considered  a 
valuable  addition  to  the  aids  to  navigation,  has  received  the  unqualified 
approbation  of  all  navigators  on  this  coast  who  are  acquainted  with  its 
operation,  and  will  probably  be  introduced  in  all  countries  where  its 
merits  are  known.  Experience  has  shown  that  it  can  be  permanently 
moored  in  deep  water,  and  that  vessels  can  safely  approach  it  within  the 
nearest  distance,  and  take  perfect  departure  from  it. 

The  Light-House  Board  has  adopted  this  buoy  as  one  of  its  permanent 
aids  to  navigation,  and  will  in  time  introduce  it  at  all  points  where  its 
presence  will  be  of  importance  to  the  navigator.  In  order  to  obtain 
reliable  data  as  to  the  operations  of  the  automatic  buoy,  Commander 
Picking  has  established  a  series  of  observations  at  all  the  stations  in  the 
neighborhood  of  the  buoys,  giving  the  time  of  hearing  it,  the  direction 
of  the  wind,  and  the  state  of  the  sea,  from  which  it  appears  that  in  the 
month  of  January,  1877,  one  of  these  buoys  was  heard  every  day  at  a 
station  one  and  one-eighth  miles  distant ;  every  day  but  two  at  one  two 
and  one-quarter  miles  distant ;  fourteen  times  at  one  seven  and  one-half 
miles  distant,  and  four  times  at  one  eight  and  one-half  miles  distant. 
It  is  heard  by  the  pilots  of  the  New  York  and  Boston  steamers  at  dis- 
tances of  from  one-fifth  to  five  miles,  and  has  been  frequently  heard  by 
the  inspector  of  the  first  light-house  district  at  a  distance  of  nine  miles, 
and  even,  under  the  most  favorable  circumstances,  fifteen  miles. 

We  sailed  around  the  buoy  and  observed  the  difference  in  the  intensity 
of  sound  in  regard  to  the  direction  of  the  wind,  which  was  at  the  time 
a  fresh  breeze  of  from  twelve  to  fifteen  miles  per  hour,  from  the  west- 
ward, the  greatest  intensity  being  apparently  at  points  forty-five  degrees 
on  either  s:de  of  the  axis  of  the  wind.  The  effect  however  was  not 
very  definitely  marked,  though  the  sound  on  the  whole  appeared  to  be 


RESEARCHES   IN   SOUND.  545 

greater  on  the  semi-circumference  of  the  circle  to  the  leeward  ;  but  the 
velocity  of  the  wind  was  so  great  that  the  noise  produced  by  it  on  the 
rigging  of  the  vessel  prevented  the  effects  from  being  definitely  observed. 

Experiments  have  been  made  with  this  buoy  carrying  whistles  of  dif- 
ferent sizes,  the  result  being  that  a  whistle  of  less  than  ten  inches  diam- 
eter does  not  give  a  sound  which  can  be  heard  as  far  as  one  of  the  latter 
size,  although  it  appears  to  the  ear  near  by  equally  loud. 

There  is  a  difference  between  the  quantity  of  sound  and  the  loudness. 
Two  sounds  may  be  equally  loud  when  heard  near  by,  yet  differ  very 
much  in  regard  to  their  being  heard  at  a  distance,  the  loudness  depend- 
ing upon  the  intensity  of  sound  or  on  the  amplitude  of  vibration  of  the 
sounding  body,  while  the  quantity  of  sound  depends  on  the  extent  of 
the  vibrating  surface. 

The  size  of  the  whistle  must  be  limited  by  the  quantity  of  air  ejected 
at  each  oscillation  of  the  buoy.  The  fact  that  the  ten-inch  whistle  gives 
a  sound  which  can  be  heard  farther  than  one  of  eight  inches  appears  to 
have  a  bearing  on  the  question  (the  actuating  force  being  the  same)  of 
the  united  effect  of  two  sounds  of  the  same  quantity  and  pitch,  since 
the  sound  from  several  parts  of  the  circumference  of  the  larger  whistle 
may  be  considered  as  a  union  of  several  sounds  of  less  quantity. 

After  these  observations  on  the  automatic  buoy  we  proceeded  along  the 
coast  to  White  Head,  at  the  entrance  of  Penobscot  Bay,  a  distance  of 
sixty  miles,  which  we  reached  at  about  twelve  o'clock  at  night,  and  cast 
anchor  in  Seal  Harbor,  near  the  White  Head  light-house. 

Our  first  operation  next  morning  was  the  examination  of  an  automatic 
fog-bell,  invented  by  Mr.  Close,  and  which  has  been  erected  by  a  special 
appropriation  of  Congress.  It  is  very  simple  in  conception,  and  would 
do  good  service  in  southern  latitudes,  where  it  would  not  be  affected  by 
the  ice.  It  consists  of  an  upright  shaft  thirty-two  feet  long,  fastened  to 
the  rock  beneath  the  water,  and  kept  in  a  vertical  position  by  a  series 
of  iron  rods  serving  as  braces.  Around  this  shaft  is  a  hollow  metallic 
float,  having  sufficient  buoyancy  by  the  motion  of  the  waves  to  elevate 
a  vertical  rod  having  at  the  upper  end  a  rack  gearing  into  a  ratchet- 
wheel.  By  means  of  projecting  pins  on  the  surface  of  the  wheel  the 
hammer  of  the  bell  is  elevated  and  the  bell  sounded  at  each  descent  of 
the  float.  This  arrangement  is  the  most  simple  and  efficient  of  any  of 
the  kind  of  which  we  have  any  knowledge. 

The  objection  to  it  is  its  liability  to  be  deranged  by  the  action  of  ice 
and  the  rusting  of  the  parts  from  exposure  to  the  weather. 

Our  next  operation  at  this  place  was  the  examination  of  the  remark- 
able abnormal  phenomenon,  which  was  the  principal  object  of  our 
excursion.  It  has  been  frequently  observed  by  the  captains  of  the 
steamers  plying  between  Boston  and  New  Brunswick,  and  has  also  been 
noticed  on  two  different  occasions  by  officers  of  the  light-house  estab- 
lishment. The  phenomenon,  as  reported  by  these  authorities,  consists 
S.  Mis.  59 35 


546  RESEARCHES    IN    SOUND. 

in  hearing  the  sound  distinctly  on  approaching  the  station  at  the  distance 
of  from  six  to  four  miles,  then  losing  it  through  a  space  of  about  three 
miles,  and  not  hearing  it  again  until  within  about  a  quarter  of  a  mile  of 
the  instrument,  when  it  becomes  suddenly  audible  in  almost  full  power. 
This  phenomenon  is  always  noticed  when  the  vessel  is  approaching  the 
signal  from  the  southwest,  and  the  wind  is  in  the  same  or  in  a  southerly 
direction,  and  therefore  opposed  to  the  direction  of  the  sound  from  the 
station,  which  is  the  case  during  a  fog.  Commander  Picking,  having 
frequently  received  complaints  from  masters  of  vessels  as  to  losing  the 
sound  at  this  place,  concluded  to  verify  the  facts  by  his  own  observation. 
For  this  purpose  he  embraced  the  opportunity  of  an  inspection-tour  in 
July,  1877,  to  approach  the  station  from  the  southwest  during  a  fog. 
In  his  own  words,  he  heard  the  sound  distinctly  through  a  space  of  from 
six  to  four  miles,  then  lost  it  and  could  hear  nothing  until  within  a 
quarter  of  a  mile  of  the  station,  when  the  blast  of  the  whistle  burst 
forth  in  full  sound.  The  wind  at  this  time  was  from  the  southward,  or 
against  the  sound.  This  cessation  in  the  hearing  of  the  sound  could 
not  have  been  due  to  the  failure  of  the  instrument  to  emit  sound,  since 
its  operation  is  automatic  when  once  started,  and  in  this  case  the  fog  so 
lifted  on  nearing  the  station  as  to  admit  the  observation  of  the  puffs  of 
steam  emitted  at  each  blast  of  the  whistle. 

On  a  previous  occasion  General  Duane  and  Mr.  Edwards  on  approach- 
ing the  same  signal  from  the  southwest  heard  the  sound  at  about  six 
miles  distance,  then  lost  it,  and  did  not  again  hear  it  until  within  about 
a  quarter  of  a  mile.  The  wind  in  this  instance  was  also  the  same  as  that 
in  the  observation  of  Commander  Picking,  namely,  from  the  southwest. 

So  well  established  was  this  phenomenon  that  General  Duane  at- 
tempted to  remedy  the  evil  by  elevating  the  duplicate  whistle  (with 
which  every  station  is  provided)  to  a  height  twenty-two  feet  above  the 
level  of  the  other  whistle,  by  placing  it  on  the  upper  end  of  a  tube. 
But  this  arrangement  produced  no  beneficial  effect. 

In  the  morning  of  September  4, 1877,  on  which  we  commenced  our  ex- 
periments, the  weather  was  clear,  the  wind  west-southwest,  the  velocity 
from  ten  to  twelve  miles,  remaining  nearly  constant  during  the  day. 
Our  first  object  was  to  verify  by  direct  observation  the  several  features  of 
the  phenomenon,  and  for  this  purpose  we  steamed  to  the  southward,  or 
directly  to  the  windward,  from  the  station  through  the  region  in  which 
the  abnormal  phenomena  had  been  noticed.  The  pressure  of  the  atmos- 
phere, as  indicated  by  an  aneroid  barometer,  was  28.9  inches.  The  tem- 
perature of  the  air  was  67°  Fahrenheit;  that  of  the  water  at  various 
points  along  our  course  was  58°,  except  at  two  points  where  the  ther- 
mometer indicated  57°.  This  difference  was  too  small  to  have  any  per- 
ceptible effect  on  the  density  of  the  rapidly  moving  air  which  was 
passing  over  the  surface  of  the  water.  As  we  increased  our  distance 
from  the  signal  the  sound  slightly  diminished  in  loudness  until  the  dis- 
tance was  between  a  quarter  and  half  a  mile,  when  it  suddenly  ceased 


RESEARCHES    IN    SOUND.  547 

to  be  heard,  and  continued  inaudible  through  a  distance  of  about  a  mile, 
when  it  was  faintly  heard  and  continued  to  increase  in  loudness  until  we 
reached  the  distance  of  four  miles ;  at  this  point  it  was  heard  with  such 
clearness  that  the  position  of  the  station  could  be  located  with  facility ; 
but  on  proceeding  farther  in  the  same  direction  it  appeared  to  diminish 
gradually  except  at  one  point,  when  a  blast,  as  indicated  by  the  steam 
issuing  from  the  whistle,  was  inaudible;  but  on  turning  the  vessel  around 
the  next  blast  was  distinctly  heard. 

As  a  second  experiment  we  retraced  the  same  line  back  to  the  station 
and  observed  the  same  phenomena  in  a  reverse  order.  The  sound  was 
heard  the  loudest  at  a  point  four  miles  from  the  station  ;  afterward  it 
diminished  and  then  became  inaudible  through  a  space  of  two  miles, 
and  then  suddenly  burst  forth  nearly  in  full  intensity  at  the  distance  of 
a  quarter  of  a  mile,  and  continued  loud  until  the  station  was  reached. 

As  a  third  experiment  the  same  line  was  traversed  again,  the  only 
difference  in  the  condition  of  the  experiment  being  that  the  whistle  on 
the  steamer  was  sounded  every  minute  between  the  blasts  of  the  signal 
at  the  station ;  and  while  the  observers  on  the  vessel  noted  the  sounds 
from  the  latter,  those  at  the  station  observed  the  sound  from  the  former. 
The  same  phenomena  as  described  in  the  previous  experiments  were 
witnessed  by  those  on  board  the  vessel,  but  on  receiving  the  report  of 
the  observers  at  the  station,  it  was  found  that  no  cessation  of  the  sound 
from  the  steamer  was  observed  through  the  whole  distance  traversed  by 
the  vessel.  It  should  be  noted  that  the  whistle  at  the  station  is  ten 
inches  in  diameter,  actuated  by  a  pressure  of  sixty  pounds  of  steam, 
and  that  on  board  the  vessel  six  inches  in  diameter  with  twenty-five 
pounds  of  steam.  It  appears  from  this  remarkable  result  that  a  feeble 
sound  passes  freely  through  what  has  been  called  the  region  of  silence 
when  sent  in  the  direction  of  the  motion  of  the  wind,  when  a  louder 
sound  does  not  pass  in  the  opposite  direction. 

As  a  fourth  experiment  the  vessel  proceeded  northward  on  the  oppo- 
site side  of  the  station  to  that  before  traversed,  but  in  the  prolongation 
of  its  previous  course.  The  sound  in  this  case  from  the  signal  to  the 
observers  on  the  vessel  was  with  the  wind,  while  that  from  the  vessel 
to  the  observers  at  the  station  was  against  the  wind.  In  this  experi- 
ment no  cessation  was  observed  on  the  vessel  in  the  hearing  of  the 
sound  from  the  station ;  it  was  heard  with  varying  intensity  to  the  dis- 
tance of  four  and  a  half  miles,  and  could  probably  have  been  heard 
much  farther  had  our  progress  not  been  interrupted  by  land.  On  return- 
ing to  the  station  the  observers  there  reported  that  after  the  vessel  had 
left  the  station  and  was  scarcely  more  than  a  hundred  yards  distant  not 
a  single  blast  of  its  whistle  was  heard.  In  this  case  the  phenomena 
which  had  been  observed  on  the  southerly  side  of  the  station  were 
exhibited  in  a  reverse  order  on  the  northerly  side. 

In  what  may  be  considered  the  fifth  experiment,  the  vessel  being  at  a 
distance  of  four  miles  from  the  station  on  the  line  traversed  in  the  first 


548  RESEARCHES   IN   SOUND. 

two  experiments,  the  sound  was  heard  slightly.  The  vessel  then  altered 
its  course  so  as  to  steam,  as  it  were,  around  the  signal,  keeping  at  the 
same  distance  until  the  direction  of  the  station  from  the  vessel  was 
nearly  at  right  angles  to  the  direction  of  the  wind  ;  at  this  point  no 
sound  was  heard  from  the  station,  although  it  had  been  slightly  heard 
at  points  along  the  curved  line  traversed  in  reaching  the  point  men- 
tioned. The  vessel  then  proceeded  toward  the  station  in  a  straight  line, 
but  no  sound  was  heard  until  it  approached  the  latter  within  one-fourth 
of  a  mile.  The  observers  at  the  station  however  heard  the  sound  from 
the  vessel  through  the  whole  distance. 

This  experiment  was  made  to  ascertain  the  truth  of  the  general  impres- 
sion that  at  this  place  the  sound  was  always  heard  better  coming  at  right 
angles  or  across  the  wind  than  in  the  direction  in  which  it  was  blowing. 
The  experiment  however  was  found  in  conformity  with  the  general  rule 
previously  established,  that  the  sound  was  usually  heard  farthest  with 
the  wind,  less  against  the  wind,  and  at  an  intermediate  distance  across 
the  wind. 

The  primary  object  of  these  investigations  is  to  determine  the  mechan- 
ical causes  to  which  the  phenomena  may  be  referred  and  from  which 
new  conclusions  may  be  deduced,  to  be  further  tested  by  experiment, 
and  such  definite  views  obtained  as  may  be  of  value  in  the  employment 
of  fog-signals  for  the  uses  of  the  mariner. 

For  this  purpose  a  number  of  different  hypotheses  may  be  provision- 
ally adopted  and  each  compared  with  the  actual  facts  observed. 

The  first  hypothesis  which  has  been  suggested  for  the  explanation  of 
the  phenomena  in  question  is  that  they  are  due  to  some  configuration  of 
the  land ;  but  on  inspecting  the  Coast-Survey  chart  of  this  region  it  will 
be  seen  that  the  nearest  land  consists  of  a  series  of  broken  surfaces  not 
rising  above  the  ocean  enough  to  reflect  sound  or  in  any  way  to  produce 
sound-shadows  in  the  region  through  which  the  phenomena  are  observed. 
This  hypothesis  therefore  is  inadmissible. 

Another  hypothesis  is  that  of  what  have  been  called  invisible  acoustic 
clouds  or  portions  of  atmosphere  existing  over  the  water  at  the  region 
of  silence,  which  might  absorb  or  variously  refract  the  sound.  That 
such  a  condition  of  a  portion  of  the  atmosphere  really  exists  in  some 
cases  is  a  fact  which  may  be  inferred  from  well-established  principles  of 
acoustics,  as  well  as  from  experimental  data.  They  would  occur  espe- 
cially in  the  case  of  dissolving  clouds,  which  would  be  accompanied  by 
local  diminutions  of  temperature,  and  also  from  portions  of  air  which 
have  been  abnormally  heated  by  contact  with  warm  earth.  But,  if  the 
phenomena  in  question  were  produced  by  a  cloud  of  this  kind,  its  pres- 
ence ought  to  be  indicated  by  transmitting  through  it  the  usual  set  of 
meteorological  instruments.  This  was  done  in  the  foregoing  experi- 
ments, but  no  change  was  observed  in  the  indications  either  of  the 
thermometer  or  barometer.  Unfortunately  we  had  not  a  hygrometer  in 
our  possession,  but  this  observation  was  less  necessary,  since  from 


RESEARCHES    IN    SOUND.  549 

abundant  testimony  it  is  established  that  the  same  phenomena  are  ex- 
hibited during  a  dense  fog,  in  which  all  parts  of  the  atmosphere  for 
miles  in  extent  must  be  in  a  homogeneous  condition.  Furthermore,  a 
local  cloud  could  not -continue  to  exist  in  a  given  space  for  more  than  an 
instant  while  a  wind  Avas  blowing  with  a  velocity  of  from  ten  to  twelve 
miles  an  hour.  Again,  this  hypothesis  fails  entirely  to  explain  the  fact 
that  this  phenomenon  is  always  observed  at  nearly  the  same  place,  espe- 
cially during  a  fog,  when  the  wind  is  in  a  southerly  direction.  Finally, 
it  is  impossible  to  conceive  of  a  cloud  so  arranged  as  a  screen  produc- 
ing a  sound-shadow  of  greater  intensity  on  one  side  than  on  the  other. 

Another  hypothesis  is  that  of  the  refraction  of  sound  due  to  the  action 
of  the  wind.  It  is  an  inference  from  well-established  theory,  as  well  as 
from  direct  observation,  that  the  sound  is  refracted  by  the  wind,  that  it 
tends  to  be  thrown  upward  when  moving  against  the  wind,  and  down- 
ward with  the  wind.  This  result  is  attributed  very  properly  to  the  dif- 
ferent velocities  of  the  strata,  that  next  the  surface  being  retarded, 
those  above  being  less  retarded. 

The  upper  part  of  the  front  of  the  wave  is  thus  thrown  backward,  and 
the  direction  of  the  wave  turned  upward.  In  the  case  of  the  experi- 
ment south  of  the  station,  the  wind  passing  over  a  long  line  of  rough 
sea  was  moving  less  rapidly  in  its  lower  stratum  than  in  the  higher,  and 
consequently  the  sound-wave  was  thrown  backward  above,  and,  as  it 
issued  from  the  instrument,  tended  to  rise  above  the  head  of  the  observer, 
and  at  a  certain  distance  from  the  origin  of  the  sound,  depending  upon 
the  difference  of  velocity  above  and  below,  was  lost  entirely  to  the  ob- 
server, and  a  sound-shadow  was  thus  produced  by  refraction  which  is 
closed  in  again  by  the  lateral  spread  of  the  sound  at  a  given  distance. 

In  the  experiment  on  the  other  side  of  the  signal,  the  vessel  proceed- 
ing to  the  north,  the  wind  coming  to  the  observer  on  the  vessel  had  to 
pass  over  a  rougher  surface  than  that  of  water,  and  consequently  the 
difference  of  velocities  above  and  below,  and  therefore  the  refraction 
would  be  greater,  and  consequently  the  sound  from  the  vessel  was  almost 
entirely  lost  to  the  observer  at  the  station,  while  the  sound  from  the 
station  was  heard  uninterruptedly  on  the  vessel,  since  it  was  moving 
with  the  wind. 

On  examining  the  records  of  experiments  of  previous  years,  I  find  a 
number  of  cases  recorded  where  sounds  were  heard  at  a  greater  dis- 
tance, while  inaudible  at  a  less  distance,  especially  one  in  connection 
with  the  fog-signal  at  Gull  Island,  in  1874.  In  this  case  the  sound,  in 
passing  from  the  signal,  was  heard  distinctly  at  the  distance  of  about 
two  miles  against  the  wind,  then  lost  for  a  space  of  about  four  and  a 
half,  and  heard  again  distinctly  for  a  distance  of  perhaps  one  mile.  At 
the  same  station,  during  the  experiments  of  1875,  the  sound  of  the 
whistles  of  the  steamers  was  heard  for  a  certain  distance,  then  ceased 
to  be  heard  for  a  considerable  interval,  and  was  then  heard  again.  Fur- 
thermore, the  pilots  of  the  steamboats  from  New  York  to  Boston  report 


550  RESEARCHES  •  IN   SOUND. 

that  the  automatic  buoy  is  found  to  intermit  its  sound,  being  heard  at 
a  distance,  then  becoming  inaudible,  and  heard  again  as  the  steamer 
approaches  the  source  of  sound. 

From  all  the  facts  which  we  have  gathered  on  this  subject,  I  think  it 
highly  probable  that  in  all  cases  in  which  sound  moving  against  the 
wind  is  thrown  up  above  the  head  of  the  observer  it  tends  to  descend 
by  the  lateral  spread  of  the  sound-wave  and  to  reach  the  earth  at  a  dis- 
tance ;  the  conditions  however  for  the  actual  production  of  this  effect 
are  somewhat  special,  and  will  depend  upon  the  amount  of  the  initial 
refraction  and  the  quantity  of  the  sound-waves.  Besides  the  lateral 
spread  of  the  sound-wave  there  are  two  other  causes  sufficient,  in  certain 
cases,  to  biing  a  portion  of  the  sound-waves  which  have  been  elevated 
in  the  air  back  again  to  the  earth :  the  first  is  when  an  upper  current 
of  wind  is  moving  in  an  opposite  or  approximately  opposite  direction 
to  that  at  the  surface  of  the  earth,  in  which  case  an  opposite  or  down- 
ward refraction  would  take  place ;  and  the  second  is  the  case  in  which 
the  surface- wind  is  terminated  above  by  strata  of  still  air ;  in  this  case, 
also,  a  reverse  refraction,  but  of  less  amount,  would  take  place,  which 
would  tend  to  bring  the  sound-wave  downward. 

We  can  readily  imagine  that  an  isolated  island,  cooled  by  the  radia- 
tion of  the  heat  by  night,  would  send  every  morning,  in  all  directions, 
a  current  of  cold  air  from  its  center.  In  this  case,  the  sound  from  a 
whistle  placed  in  the  center  of  the  island  would  be  inaudible  in  a  space 
entirely  surrounding  it,  and  thus  give  rise  to  a  condition  mentioned  by 
General  Duane,  in  which  a  fog-signal  appeared  to  be  surrounded  by  a 
belt  of  silence. 

The  next  experiment  was  made  on  the  morning  of  the  5th,  on  leaving 
the  station.  In  this  case  we  proceeded  along  the  direction  of  the  same 
line  in  which  the  first,  second,  and  third  experiments  were  made  the  day 
before.  The  wind  had  changed  about  four  points  to  the  southward. 
As  in  the  preceding  experiments,  the  sound  was  lost  again  at  the  dis- 
tance of  about  one-fourth  of  a  mile,  but  was  not  distinctly  regained, 
though  some  of  the  observers  thought  they  heard  it  at  a  distance  of  two 
and  one-half  miles. 

The  only  perceptible  difference  in  the  wind  on  the  5th  was  that  it  was 
a  little  less  rapid,  and  four  points  more  to  the  southward. 

From  a  subsequent  report  of  the  keepers,  the  whistle  of  the  vessel 
was  heard  continuously  as  far  as  the  puffs  of  steam  could  be  observed, 
a  distance  six  or  seven  miles.  In  this  case  the  sound  was  moving  with 
the  wind.  These  results  therefore  are  in  accordance  with  those  pre- 
viously obtained. 

The  next  experiments  were  made  at  Monhegan,  an  island  sixteen  miles 
southwest  of  White  Head.  On  this  island  there  is  a  Daboll  trumpet 
actuated  by  a  hot-air  engine.. 

We  departed  from  this  station  in.  a  westerly  direction  at  an  angle  of 


RESEARCHES   IN   SOUND.  551 

45°  to  the  right  of  the  direction  of  the  wind,  and  after  proceeding  about 
one  mile,  as  estimated  by  time,  we  lost  the  sound  of  the  signal.  We 
then  turned  at  right  angles  to  our  former  course  and  proceeded  toward 
the  leeward,  keeping  about  the  same  distance  from  the  signal,  when  the 
sound  was  regained  at  a  point  which  probably  depended  upon  the  direc- 
tion of  the  wind  and  the  axis  of  the  trumpet  combined.  From  this  point 
it  was  heard  to  a  point  to  the  leeward,  and  thence  we  retraced  our 
course  at  about  the  same  distance  and  proceeded  across  the  axis  of  the 
trumpet  toward  the  windward,  where  the  sound  was  again  lost.  The 
only  definite  result  from  this  experiment  was  another  case  of  the  sound 
being  heard  farther  to  the  leeward  than  to  the  windward. 

After  this  experiment  we  returned  to  Portland. 

An  interesting  fact  may  be  mentioned  in  connection  with  this  station, 
having  a  bearing  upon  the  protection  of  light-houses  from  lightning. 
The  fog- signal  is  placed  on  a  small  island  separated  from  the  large  island 
by  a  water-space  of  about  one-eighth  of  a  mile.  General  Duane,  desir- 
ing to  connect  the  light-house  and  fog-signal  by  an  electrical  communi- 
cation, suspended  a  wire  between  the  two  points  and  attempted  to  form 
a  ground  connection  by  depositing  a  plate  of  metal  in  the  ground  on 
each  island,  but  to  his  surprise,  though  the  arrangements  were  made  by 
a  skilled  telegrapher,  no  signal  would  pass.  The  two  islands  being 
composed  of  rock  and  the  soil  limited  in  thickness,  the  conduction  was 
imperfect,  and  it  was  only  by  plunging  the  plate  of  metal  into  the  water 
on  each  side  of  the  space  between  the  two  islands  that  a  signal  could 
be  transmitted. 

No  further  experiments  on  sound  were  made  during  this  excursion, 
because  the  vessel  could  no  longer  be  spared  from  more  pressing  light- 
house duty  in  the  way  of  inspection  and  the  stated  supply  of  materials 
to  the  stations. 

On  my  return  to  New  York,  accompanied  by  Mr.  Woodruff,  J  took  the 
route  by  the  Western  Railway  to  the  Hudson  Eiver  at  Troy.  This  line 
was  chosen  in  order  to  make  some  investigations  relative  to  any  pecu- 
liarities of  sound  which  might  be  observed  in  the  Hoosac  tunnel,  through 
which  the  railroad  passes.  For  this  purpose  we  spent  a  day  at  East 
Windsor,  a  village  situated  near  the  west  end  of  thje  tunnel,  and  were 
very  cordially  received  by  the  engineers  in  charge. 

The  tunnel  is  four  and  three-quarters  miles  in  length,  twenty -four  feet 
wide,  and  twenty  feet  high  to  the  crown  of  the  arch.  It  ascends  slightly 
from  either  end  to  a  point  near  the  center,  where  there  is  a  ventilating- 
shaft  1,028  feet  high  extending  to  the  outer  air  above.  In  winter,  when 
the  external  temperature  is  less  than  that  within  the  tunnel,  there  is  a 
constant  current  from  each  end  toward  the  center,  and  in  the  summer, 
when  the  temperatures  are  reversed,  there  is  a  current  out  of  the  tunnel 
at  either  end,  except  when  the  external  wind  is  sufficiently  strong,  es- 
pecially from  the  west,  to  reverse  the  direction  of  the  current  from  one 


552  RESEARCHES   IN   SOUND. 

half  and  direct  tbe  stream  entirely  out  of  the  other  entrance.  At  the 
time  of  our  visit,  there  was  a  gentle  current  flowing  out  of  both  ends. 

The  only  peculiarity  of  sound  which  had  been  observed,  as  stated  by 
the  engineers,  was  that  it  was  greatly  stifled  at  the  time  by  the  smoke 
with  which  the  air  was  filled  immediately  after  the  passage  of  a  loco- 
motive. So  great  was  this  in  some  cases  that  accidents  were  imminent 
to  the  workmen,  who  are  constantly  occupied  in  the  tunnel  in  lining  the 
crown  of  the  arch  with  brick,  by  the  sudden  appearance  of  a  locomo- 
tive, the  approach  of  which  had  not  been  heard. 

That  the  audibility  of  sound  should  be  diminished  by  smoke  was  so 
contrary  to  previous  conceptions  on  the  subject,  since  sound  is  not  prac- 
tically interrupted  by  fog,  snow,  rain,  or  hail,  that  I  was  induced  to 
attribute  the  effects  which  had  been  observed  to  another  cause,  and  to 
regard  the  phenomenon  as  due  to  an  exaggerated  flocculent  condition 
of  the  air  in  the  tunnel ;  adopting  in  this  instance  the  hypothesis  ad- 
vanced by  Dr.  Tyndall,  and  so  well  illustrated  by  his  ingenious  ex- 
periments. The  effect  which  would  be  produced  in  the  condition  of 
the  air  in  the  tunnel  by  the  passage  of  a.  locomotive  is  indicated  by 
the  appearance  of  the  emitted  steam  extending  behind  the  smoke- 
stack of  a  locomotive  in  rapid  progress  before  the  observer  at  a  dis- 
tance. This  consists  of  a  long  stream  composed  of  a  series  of  globu- 
lar masses  produced  by  the  successive  puffs  of  steam  which  are  emitted 
at  equal  intervals.  Allowing  the  diameter  of  the  driving-wheels  to  be 
five  feet,  then  since  four  puffs  are  made  at  each  revolution  of  the  wheels, 
a  puff  of  hot  steam  would  be  given  out  at  every  four  feet  travelled  by 
the  engine,  and  these  puffs  mingling  with  the  air  at  the  ordinary  tem- 
perature would  produce  an  exaggerated  iiocculeut  condition.  On  our 
expressing  a  desire  to  witness  the  effect  upon  sound  of  the  passage  of  a 
locomotive  through  the  tunnel,  Mr.  A.  W.  Locke,  one  of  the  engineers 
who  had  charge  of  the  western  section,  politely  offered  us  the  means  of 
experimenting  on  this  point,  and  also  of  passing  leisurely  through  the 
tunnel  on  a  hand-car. 

To  observe  the  effect  of  a  locomotive  on  the  sound  we  took  advantage 
of  the  entrance  of  a  freight  train,  impelled  by  two  engines,  the  extra 
one  being  necessary  to  drive  the  load  up  the  inclined  plane  to  the  mid- 
dle of  the  tunnel,  where  it  was  detached  and  returned  along  the  same 
line,  while  the  train  was  drawn  the  remaining  distance  along  the  eastern 
decline  by  a  single  engine.  In  order  to  make  the  experiment  with  re- 
gard to  sound  the  time  was  accurately  noted  during  which  the  noise  of 
the  entering  engines  could  be  distinctly  heard,  which  would  give  ap- 
proximately the  distance  the  sound  travelled  through  the  flocculeut  at- 
mosphere produced  by  the  locomotive  before  becoming  inaudible,  and 
again  the  time  was  noted  from  the  first  hearing  of  the  returning  engine 
until  it  reached  the  end  of ^  the  tunnel.  In  the  mean  time  the  current  of 
air  blowing  through  the  tunnel  had  removed  a  considerable  portion,  at 
least,  of  the  flocculent  atmosphere,  so  that  the  seund  in  this  case  came 


RESEARCHES   IN   SOUND.  553 

through  an  atmosphere  of  comparative  uniformity  of  temperature,  or 
one  much  less  flocculent  than  the  other ;  the  result  was  that  the  dura- 
tion of  sound  in  the  first  case  was  about  a  minute,  while  in  the  second 
it  was  upward  of  two  minutes.  The  darkness  in  the  tunnel,  on  account 
of  the  smoke,  was  so  profound  immediately  after  the  passage  of  a  loco- 
motive, that  with  two  large  torches,  charged  with  mineral  oil,  the  sides 
of  the  tunnel  at  a  distance  of  six  feet  could  scarcely  be  observed ;  while 
in  the  other  half  of  the  tunnel,  where  no  smoke  existed,  the  eastern 
opening  could  be  observed  like  a  star  in  the  distance  of  upward  of  two 
miles.  It  was  therefore  not  surprising  that  the  stifling  of  the  sound 
which  was  observed  should  be  referred  to  the  smoke  as  a  palpable  cause, 
and  that  the  more  efficient  one  of  the  varying  density  or  flocculent  con- 
dition should  be  disregarded. 

The  method  of  determining  by  experiment  the  question  as  to  which  of 
these  causes  was  the  efficient  one  did  not  occur  to  me  until  we  had  left 
the  tunnel,  and  then  the  simple  expedient  suggested  itself  to  me,  for  the 
purpose  of  repeating  the  experiment,  that  instead  of  locomotives  charged 
with  wood,  two  locomotives  charged  with  charcoal  or  coke — which  emit 
no  smoke,  but  only  transparent  gases  principally  carbonic  acid — should 
be  used  in  an  experiment  similar  to  the  one  just  described.  This  ex- 
periment Mr.  Locke  has  kindly  promised  to  perform  as  soon  as  a  con- 
venient opportunity  shall  occur. 

The  opportunity  was  embraced  while  at  the  mouth  of  the  tunnel  to 
make  some  observations  which  might  have  a  bearing  uj>on  the  phenom- 
ena of  the  aerial  echo.  For  this  purpose,  a  d vantage  was  taken  of  a 
large  tool-chest,  which  happened  to  be  place  d  about  twenty  or  thirty 
feet  within  the  western  mouth  of  the  tunnel.  By  slamming  down  vio- 
lently the  cover  of  this  chest,  a  loud  sound  of  an  explosive  character 
was  produced,  from  which  a  prolonged  echo  was  returned  from  the  in- 
terior of  the  tunnel.  This  echo  was  slightly  intermittent,  suddenly  in- 
creasing in  loudness  at  intervals  for  a  moment,  and  again  resuming  its 
uniform  intensity.  This  effect  was  attributed  to  projecting  pieces  of 
rock  in  that  part  of  the  tunnel  which  had  not  been  lined  with  brick. 
An  echo  was  however  evidently  returned  from  that  portion  of  which 
the  sides  were  not  projecting,  which  I  would  consider  an  effect  of  the 
same  cause  which  produces  the  aerial  echo. 

AERIAL  ECHOES. 

During  the  year  1877,  (as  also  in  187G,)  series  of  experiments  were 
made  on  the  aerial  echo,  in  which  I  was  assisted — in  the  first  series  by 
General  Woodruff,  engineer  of  the  third  light-house  district, — and  in  the 
second  series  by  Edward  Woodruff,  assistant  engineer  of  the  same  dis- 
trict. These  experiments  were  made  principally  at  Block  Island,  but 
also  at  Little  Gull  Island.  Especial  attention  has  been  given  to  this 
phenomenon,  which  consists  in  a  distinct  echo  from  the  verge  of  the 
horizon  in  the  direction  of  the  prolongation  of  the  axis  of  the  trumpet 


554  RESEARCHES    IN    SOUND. 

of  the  siren ,  "because  the  study  of  it  has  been  considered  to  offer  the 
easiest  access  to  the  solution  of  the  question  as  to  the  cause  of  all  the 
abnormal  phenomena  of  sound,  and  also  because  it  is  in  itself  an  object 
of  much  scientific  interest. 

In  my  previous  notice  of  this  phenomenon,  in  the  report  of  the  light- 
house board  for  1874, 1  suggested  that  it  might  be  due  to  the  reflection 
from  the  crests  of  the  waves  of  the  ocean ;  but  as  the  phenomenon  has 
been  observed  during  all  conditions  of  the  surface  of  the  water,  this  ex- 
planation is  not  tenable. 

Another  hypothesis  has  been  suggested,  that  it  is  due  to  a  flocculent 
condition  of  the  atmosphere,  or  to  an  acoustic  invisible  cloud,  of  a 
density  in  different  parts  differing  from  that  of  the  general  atmosphere 
at  the  time.  To  test  this  hypothesis  experimentally,  the  large  trumpet 
of  the  siren  was  gradually  elevated  from  its  usual  horizontal  position  to 
a  vertical  one.  In  conception,  this  experiment  appears  very  simple,  but 
on  account  of  the  great  weight  of  the  trumpet,  it  required  the  labor  of 
several  men  for  two  days  to  complete  the  arrangements  necessary  to  the 
desired  end.  The  trumpet,  in  its  vertical  position,  was  sounded  at  in- 
tervals for  two  days,  but  in  no  instance  was  an  echo  heard  from  the 
zenith,  but  one  was  in  every  case  produced  from  the  entire  horizon. 
The  echo  appeared  to  be  somewhat  louder  from  the  land  portion  of  the 
circle  of  the  horizon  than  from  that  of  the  water.  On  restoring  the 
trumpet  to  its  horizontal  position,  the  echo  gradually  increased  on  the 
side  of  the  water,  until  the  horizontal  position  was  reached,  when  the 
echo,  as  usual,  appeared  to  proceed  from  an  angle  of  about  twenty  de- 
grees of  the  horizon,  the  middle  of  which  was  in  the  prolongation  of 
the  axis  of  the  trumpet.  A  similar  experiment  was  made  with  one  of 
the  trumpets  of  the  two  sirens  at  Little  Gull  Island.  In  this  case  the 
trumpet  was  sounded  in  a  vertical  position  every  day  for  a  week  with 
the  same  result.  On  one  occasion  it  happened  that  a  small  cloud  passed 
directly  over  the  island  on  which  the  light-house  is  erected,  and  threw 
down  on  it  a  few  drops  of  rain.  At  the  moment  of  the  passage  of  this 
cloud  the  trumpet  was  sounded,  but  no  echo  was  produced. 

From  these  experiments  it  is  evident  that  the  phenomenon  is  in  some 
way  connected  with  the  plane  of  the  horizon,  and  that  during  the  contin- 
uance of  the  experiment  of  sounding  the  trumpets  while  directed  toward 
the  zenith  no  acoustic  cloud  capable  of  producing  reflection  of  sound  ex- 
isted in  the  atmosphere  above  them. 

Another  method  of  investigating  this  phenomenon  occurred  to  me, 
which  consisted  in  observing  the  effects  produced  on  the  ears  of  the  ob- 
server by  approaching  the  origin  of  the  echo.  For  this  purpose,  during 
the  sounding  of  the  usual  interval  of  twenty  seconds  of  the  large  trumpet 
at  Block  Island,  observations  were  made  from  a  steamer  which  proceeded 
from  the  station  into  the  region  of  the  echo  and  in  the  line  of  the  pro- 
longation of  the  axis  of  the  trumpet,  with  the  following  results  : 

1.  As  the  steamer  advanced,  and  the  distance  from  the  trumpet  was 


RESEARCHES   IN   SOUND.  555 

increased,  the  londness  of  the  echo  diminished ,  contrary  to  the  effect  of 
an  echo  from  a  plane  surface,  since  in  the  latter  case  the  echo  would 
have  increased  in  loudness  as  the  reflecting  surface  was  approached, 
because  the  whole  distance  travelled  by  the  sound-wave  to  and  from  the 
reflector  would  have  been  lessened.  The  effect  however  is  in  accord- 
ance with  the  supposition  that  the  echo  is  a  multiple  sound,  the  several 
parts  of  which  proceed  from  different  points  at  different  distances  of  the 
space  in  front  of  the  trumpet,  and  that  as  the  steamer  advances  toward 
the  verge  of  the  horizon,  it  leaves  behind  it  a  number  of  the  points  from 
which  the  louder  ones  proceed,  and  thus  the  effect  upon  the  ear  is  dimin- 
ished as  the  distance  from  the  trumpet  is  increased. 

2.  The  duration  of  the  echo  was  manifestly  increased,  in  one  instance, 
from  five  seconds,  as  heard  at  the  mouth  of  the  trumpet,  to  twenty 
seconds. 

This  would  also  indicate  that  the  echo  is  a  multiple  reaction  of  vary- 
ing intensities  from  different  points,  and  that  at  the  place  of  the  steamer 
the  fainter  ones  from  a  greater  distance  would  be  heard,  which  would 
be  inaudible  near  the  trumpet. 

3.  The  arc  of  the  horizon  from  which  the  echo  appeared  to  come  was 
also  increased  in  some  cases  to  more  than  three  times  that  subtended  by 
the  echo  at  the  place  of  the  trumpet.    This  fact  again  indicates  that  the 
echo  consists  of  multiple  sounds  from  various  points  at  or  near  the  sur- 
face of  the  sea,  the  angle  which  the  aggregate  of  these  points  subtend 
necessarily  becoming  greater  as  the  steamer  advances. 

But  perhaps  the  most  important  facts  in  regard  to  the  echo  are  those 
derived  from  the  series  of  observations  on  the  subject,  made  by  Mr. 
Henry  W.  Clark,  the  intelligent  keeper  of  the  principal  light-house  sta- 
tion on  Block  Island,  and  by  Joseph  Whaley,  keeper  of  the  Point  Judith 
light-house.  Mr.  Clark  was  furnished  with  a  time-marker  to  observe  the 
duration  of  the  echo,  and  both  were  directed  to  sound  the  trumpets  every 
Monday  morning  for  half  an  hour,  noting  the  temperature,  the  height  of 
the  barometer,  the  state  of  the  weather  as  to  clearness  or  fog,  the  direc- 
tion and  intensity  of  the  wind,  and  the  surface  of  the  ocean. 

From  the  observations  made  at  these  two  points,  for  more  than  two 
years  at  one  station  and  over  a  year  at  the  other,  the  echo  may  be  con- 
sidered as  produced  constantly  under  all  conditions  of  weather,  even 
during  dense  fogs,  since  at  Block  Island  it  was  heard  106  times  out  of 
113,  and  at  Point  Judith  50  times  out  of  57,  and  on  the  occasions  when 
it  was  not  heard  the  wind  was  blowing  a  gale,  making  a  noise  sufficiently 
intense  to  drown  the  sound  of  the  echo.  These  results  appear  to  be 
sufficient  to  disprove  the  hypothesis  that  the  phenomenon  is  produced 
by  an  acoustic  cloud  accidentally  situated  in  the  prolongation  of  the  axis 
of  the  trumpet.  It  must  be  due  to  something  more  permanent  in  its 
effects  than  that  from  a  portion  of  air  differing  from  that  of  the  general 
atmosphere  in  temperature  or  density,  since  such  a  condition  cannot 
exist  in  a  dense  fog  embracing  all  the  region  of  the  locality  of  the  phe- 


556  RESEARCHES   IN   SOUND. 

iioinenon.  Indeed,  it  is  difficult  to  conceive  how  the  results  can  be  pro- 
duced, even  in  a  single  instance,  from  a  flocculent  portion  of  atmosphere 
in  the  prolongation  of  the  axis  of  the  trumpet,  since  a  series  of  patches 
of  clouds  of  different  temperature  and  densities  would  tend  to  absorb  or 
stifle  by  repeated  reflections  a  sound  coming  from  their  interior  rather 
than  to  transmit  it  to  the  ear  of  the  observer. 

The  question,  therefore,  remains  to  be  answered :  what  is  the  cause 
of  the  aerial  echo  ?  As  I  have  stated,  it  must  in  some  way  be  connected 
with  the  plane  of  the  horizon.  The  only  explanation  which  suggests 
itself  to  me  at  present  is  that  the  spread  of  the  sound  which  fills  the 
whole  atmosphere  from  the  zenith  to  the  horizon  with  sound-waves 
may  continue  their  curvilinear  direction  until  they  strike  the  surface 
of  the  water  at  such  an  angle  and  direction  as  to  be  reflected  back  to 
the  ear  of  the  observer.  In  this  case  the  echo  would  be  heard  from  a 
perfectly  flat  surface  of  water,  and  as  different  sound-rays  would  reach 
the  water  at  different  distances  and  from  different  azimuths,  they  would 
produce  the  prolonged  character  of  the  echo  and  its  angular  extent  along 
the  horizon. 

While  we  do  not  advance  this  hypothesis  as  a  final  solution  of  the 
question,  we  shall  provisionally  adopt  it  as  a  means  of  suggesting  further 
experiments  in  regard  to  this  perplexing  question  at  another  season. 

GENERAL  CONCLUSIONS. 

From  all  the  experiments  which  have  been  made  by  the  Light-House 
Board  in  regard  to  the  transmission  of  sound  in  free  air  and  those  de- 
rived from  other  observations  which  can  be  fully  relied  upon,  the  follow- 
ing conclusions  may  be  considered  established,  subject  however  to  such 
further  modification  and  extension  as  subsequent  investigation  may 
seem  to  indicate: 

1.  The  audibility  of  sound  at  a  distance  (the  state  of  the  atmosphere 
being  constant)  depends  upon  the  character  of  the  sound.  The  distance 
through  which  a  sound  may  be  heard  is  governed  by  the  pitch,  the  loud- 
ness,  and  the  quantity  of  sound.  The  pitch  or  frequency  of  the  impulses 
in  a  given  time  must  not  be  too  high,  otherwise  the  amplitude  of  vibra- 
tion will  be  too  small  to  allow  a  sufficient  quantity  of  air  to  be  put  into 
motion ;  neither  must  the  pitch  be  too  low,  for  in  this  case  the  motion  of 
the  atoms  of  air  in  the  sound-wave  will  not  be  sufficiently  rapid  to  con- 
vey the  impulse  to  a  great  distance.  Again,  the  greater  the  loudness 
of  the  sound,  which  depends  upon  the  amplitude  of  the  vibrations  of  the 
sounding-body,  the  greater  will  be  the  distance  at  which  it  will  be  heard. 
And  finally,  the  greater  the  quantity  of  sound,  which  depends  upon  the 
magnitude  of  the  vibrating  surface,  the  greater  will  be  the  distance  to 
which  it  is  audibly  transmitted.  These  results  are  derived  from  observa- 
tions on  the  siren,  the  reed-trumpet,  and  the  automatic  buoy.  The  effect 
of  quantity  of  sound  is  shown  in  the  fact  that  in  sounding  different  in- 


RESEARCHES    IN    SOUND.  557 

struments  at  the  same  time,  it  was  found  that  two  sounds  apparently  of 
the  same  loudness  were  heard  at  very  different  distances. 

2.  The  audibility  of  sound  depends  upon  the  state  of  the  atmosphere. 
A  condition  most  favorable  to  the  transmission  of  sound  is  that  of  per- 
fect stillness  and  uniform  density  and  temperature  throughout.    This 
is  shown  by  the  observations  of  Parry  and  other  Arctic  explorers  ;  al- 
though in  this  case  an  efficient  and  co-operating  cause  is  doubtless  the 
downward  refraction  of  sound  due  to  the  greater  coldness  of  the  lower 
strata  of  air,  as  first  pointed  out  by  Professor  Eeynolds.    Air  however  is 
seldom  in  a  state  of  uniform  density,  but  is  pervaded  by  local  currents, 
due  to  contact  with  portions  of  the  earth  unequally  heated,  and  from 
the  refractions  and  reflections  to  which  the  sound-wave  is  subjected  in 
its  passage  through  such  a  medium  it  is  broken  up  and  lost  to  the  ear 
at  a  less  distance. 

3.  But  the  most  efficient  cause  of  the  loss  of  audibility  is  the  direct 
effect  produced  by  the  wind.    As  a  general  rule,  a  sound  is  heard  far- 
ther when  moving  with  the  wind  than  when  moving  against  it.    This 
effect,  which  is  in  conformity  with  ordinary  observation,  is  not  due  to  an 
increase  of  velocity  of  the  sound-wave  in  one  direction  and  a  diminution  in 
the  other  by  the  motion  of  the  wind  except  in  an  imperceptible  degree;  for 
since  sound  moves  at  the  rate  of  about  seven  hundred  and  fifty  miles  an 
hour,  a  wind  of  seven  miles  and  a  half  an  hour  could  increase  or  dimin- 
ish the  velocity  of  the  sound-wave  only  one  per  cent,  while  the  effect  ob- 
served is  in  some  cases  several  hundred  per  cent.    It  is  however  due  to 
a  change  in  its  direction.     Sound  moving  with  the  wind  is  refracted  or 
thrown  down  toward  the  earth ;  while  moving  against  the  wind  it  is 
refracted  upward  and  passes  over  the  head  of  the  observer,  so  as  to  be 
heard  at  a  distance  at  an  elevation  of  several  hundred  feet  when  in- 
audible at  the  surface  of  the  earth. 

4.  Although,  as  a  general  rule,  the  sound  is  heard  farther  when  mov- 
ing with  the  wind  than  when  moving  against  it,  yet  in  some  instances 
the  sound  is  heard  farthest  against  the  wind ;  but  this  phenomenon  is 
shown  to  be  due  to  a  dominant  upper  wind,  blowing  at  the  time  in  an 
opposite  direction  to  that  at  the  surface  of  the  earth.    These  winds  are 
not  imaginary  productions  invented  to  explain  the  phenomena,  but 
actual  existences,  established   by  observation,  as  in  the  case  of  the 
experiments  made  at  Sandy  Hook,  in  1874,  by  means  of  balloons,  and 
from  the  actual  motion  of  the  air  in  the  case  of  northeast  storms,  as 
observed  at  stations  on  the  coast  of  Maine. 

5.  Although  sound  issuing  from  the  mouth  of  a  trumpet  is  at  first  con- 
centrated in  a  given  direction,  yet  it  tends  to  spread  so  rapidly  that  at 
the  distance  of  three  or  four  miles  it  fills  the  whole  space  of  air  inclosed 
within  the  circuit  of  the  horizon,  and  is  heard  behind  the  trumpet  nearly 
as  well  as  at  an  equal  distance  in  front  of  its  mouth.    This  fact  precludes 
the  use  of  concave  reflectors  as  a  means  of  increasing  the  intensity  of 
sound  in  a  given  direction  j  for  although  at  first  they  do  give  an  increase 


558  RESEARCHES   IN   SOUND. 

of  sound  in  the  direction  of  the  axis,  it  is  only  for  a  comparatively  short 
distance. 

6.  It  lias  been  established,  contrary  to  what  has  formerly  been  thought 
to  be  the  case,  that  neither  fog,  snow,  hail,  nor  rain,  materially  interferes 
with  the  transmission  of  loud  sounds.    Tlje  siren  has  been  heard  at  a 
greater  distance  during  the  prevalence  of  a  dense  and  widely-extended 
fog  than  during  any  other  condition  of  the  atmosphere.    This  may 
however  be  attributed  to  the  uniform  density  and  stillness  of  the  air  at 
the  time. 

7.  In  some  cases  sound-shadows  are  produced  by  projecting  portions 
of  land  or  by  buildings   situated  near  the  origin  of  the  sound,  but 
these  shadows  are  closed  in  by  the  spread  of  the  sound-waves,  and  thus 
exhibit  the  phenomenon  of  sound  being  heard  at  a  distance  and  after- 
wards lost  on  a  nearer  approach  to  the  station. 

8.  It  frequently  happens  on  a  vessel  leaving  a  station,  that  the  sound 
is  suddenly  lost  at  a  point  in  its  course,  and  after  remaining  inaudible  some 
time,  is  heard  again  at  a  greater  distance,  and  is  then  gradually  lost  as 
the  distance  is  farther  increased.    This  phenomenon  is  only  observed 
when  the  sound  is  moving  against  the  wind,  and  is  therefore  attributed 
to  the  upward  refraction  of  the  sound-wave,  which  passes  over  the  head 
of  the  observer  and  continues  an  upward  course  until  it  nearly  reaches 
the  upper  surface  of  the  current  of  wind,  when  the  refraction  will  be  re- 
versed and  the  sound  sent  downward  to  the  earth  j  or  the  effect  may  be 
considered  as  due  to  a  sound-shadow  produced  by  refraction,  which  is 
gradually  closed  in  at  a  distance  by  the  lateral  spread  of  the  sound- 
wave near  the  earth,  on  either  side,  in  a  direction  which  is  not  affected 
by  the  upward  refraction.    Another  explanation  may  be  found  in  the 
probable  circumstance  of  the  lower  sheet  of  sound-beams  being  actually 
refracted  into  a  serpentine  or  undulating  course,  as  suggested  in  the 
Appendix  to  the  Report  of  the  Light-House  Board  for  1875.     (See  page 
513.)     Such  a  serpentine  course  would  result  from  successive  layers  of 
unequal  velocity  in  an  opposing  wind ;  as  being  retarded  at  and  near 
the  surface  of  the  earth,  attaining  its  maximum  velocity  at  a  height  of 
a  few  hundred  feet,  and  then  being  again  retarded  at  greater  elevations, 
by  the  friction  of  upper  counter  currents  or  stationary  air.     In  some 
cases  the  phenomenon  is  due  to  one  or  the  other  of  these  causes,  and  in 
other  cases  to  all  combined.    That  it  is  not  due  to  the  obstructing  or 
screening  effects  of  an  abnormal  condition  of  the  atmosphere  is  shown 
by  the  fact  that  a  sound  transmitted  in  an  opposite  direction,  through 
what  is  called  the  region  of  silence,  passes  without  obstruction.    It  is 
probable  from  all  the  observations,  that  in  all  cases  of  refraction  of  a 
sound  moving  against  the  wind  it  tends  again  to  descend  to  the  earth 
by  the  natural  spread  of  the  sound. 

9.  The  existence  of  a  remarkable  phenomenon  has  been  established, 
which  is  exhibited  in  all  states  of  the  atmosphere  during  rain,  snow,  and 
dense  fog,  to  which  has  been  given  the  name  of  aerial  echo.    It  consists 


RESEARCHES   IN   SOUND.  559 

of  a  distinct  echo,  apparently  from  a  space  near  the  horizon  of  fifteen  or 
twenty  degrees  in  azimuth,  directly  in  the  prolongation  of  the  axis  of 
the  trumpet.  The  loudness  of  this  echo  depends  upon  the  loudness  and 
quantity  of  the  original  sound,  and  therefore  it  is  produced  with  the 
greatest  distinctness  by  the  siren.  It  cannot  be  due  to  the  accidental 
position  of  a  flocculent  portion  of  atmosphere,  nor  to  the  direct  reflection 
from  the  crests  of  the  waves,  as  was  at  first  supposed,  since  it  is  always 
heard  except  when  the  wind  is  blowing  a  hurricane. 

As  a  provisional  explanation,  the  hypothesis  has  been  adopted  that 
in  the  natural  spread  of  the  waves  of  sound,  some  of  the  rays  must  take 
such  a  curvilinear  course  as  to  strike  the  surface  of  the  water  in  an 
opposite  direction  and  thus  be  reflected  back  to  the  station  or  location 
of  the  origin  of  the  sound. 

LIGHT-HOUSE  BOARD,  October,  1877. 


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